Lattice dynamics across the ferroelastic phase transition in Ba2ZnTeO6: a Raman and first-principles study.

Structural phase transitions drive several unconventional phenomena including some illustrious ferroic attributes which are relevant for technological advancements. On this note, we have investigated the ferroelastic structural transition of perovskite-type trigonal Ba2ZnTeO6 across Tc ∼ 150 K. With the help of Raman spectroscopy and density-functional theory (DFT)-based calculations, we report new intriguing observations associated with the phase transition in Ba2ZnTeO6. We observed the presence of a central peak (quasi-elastic Rayleigh profile), huge softening in the soft mode, hysteretic phonon behavior, and signatures of coexistent phases. The existence of a central peak in Ba2ZnTeO6 is manifested by a sharp rise in the intensity of the Rayleigh profile concomitant with the huge damping (or softening) of the soft mode (at ∼31 cm-1) near Tc, shedding light on the lattice dynamics during the phase transition. This is further corroborated by our phonon calculations that show that the soft mode (Eg) in the high-symmetry structure involving TeO6 octahedral rotation (with Ba and Zn translation) condenses into Ag and Bg modes in the C2/m low-symmetry phase. While most of the phonon bands split below Tc confirming the phase transition, we observed thermal hysteretic behavior of phonon modes, which signifies the first-order nature of the transition and the presence of coexisting phases as corroborated by our temperature-dependent X-ray diffraction and specific heat measurements.

Corneal Biomechanics and Intraocular Pressure Following Scleral Lens Wear in Penetrating Keratoplasty and Keratoconus.

OBJECTIVE: To compare corneal biomechanics and intraocular pressure (IOP) in keratoconus and penetrating keratoplasty eyes before and after nonfenestrated scleral lens wear. METHODS: Twenty-three participants were enrolled, and 37 eyes were included in the analysis (11 penetrating keratoplasty and 26 keratoconus). A range of corneal biomechanical parameters and IOP were measured using the CORVIS ST before and after 8 hr of nonfenestrated scleral lens wear (Keracare, Acculens, Denver, CO). RESULTS: Before lens wear, penetrating keratoplasty eyes displayed significantly greater median values for central corneal thickness (97 µm thicker, P=0.02), IOP (3.89 mm Hg higher, P=0.01), and biomechanical parameter A2 length (0.48 mm longer, P=0.003) compared with keratoconic eyes. No significant changes in corneal biomechanical parameters or IOP were observed after scleral lens wear in either group (all P>0.05). CONCLUSION: Although nonfenestrated scleral contact lenses can induce a subatmospheric pressure after lens settling and compress tissue surrounding the limbus, no significant changes were detected in the corneal biomechanical parameters studied using CORVIS ST after scleral lens wear in eyes with penetrating keratoplasty and keratoconus.

The effect of scleral lenses on vision, refraction and aberrations in post-LASIK ectasia, keratoconus and pellucid marginal degeneration.

PURPOSE: To quantify the effect of a single scleral lens design on visual acuity and ocular higher-order aberrations in eyes with post-LASIK ectasia, keratoconus and pellucid marginal degeneration (PMD) that could not achieve satisfactory vision with spectacles or soft contact lenses. METHODS: Forty-six eyes of 28 participants fitted with diagnostic scleral lenses (KeraCare) were analysed, including 19, 15 and 12 eyes with post-LASIK ectasia, keratoconus and PMD, respectively. Corrected distance visual acuity (CDVA) and ocular aberrations were measured prior to lens wear and during lens wear after 60 min of settling. An i-Trace aberrometer was used to determine aberrations over a 4.5 mm diameter pupil. RESULTS: Before lens wear, the median (95% confidence interval) values across all groups were: CDVA 0.30 (0.30, 0.40) logMAR, spherical equivalent refraction -2.75 (-5.25, -2.12) D, cylindrical refraction 3.75 (2.50, 5.00) D, higher-order-root-mean-square error (HO-RMS) 0.90 (0.64, 1.03) µm and vertical coma co-efficient C(3,-1) -0.32 (-0.42, -0.12) µm. RMS coma of 0.52 (0.40, 0.74) µm was higher for the keratoconus group than for the other groups (p < 0.05). During lens wear, values improved considerably across all groups: CDVA 0.0 (0.0, 0.00) logMAR, spherical equivalent refraction -0.50 (-0.75, +0.50) D, cylindrical refraction 0.50 (0.00, 0.50) D, HO-RMS 0.32 (0.26, 0.42) µm and C(3,-1) +0.12 (+0.02, +0.19) µm (all p < 0.001 compared to pre-lens wear). While reduced significantly, RMS coma remained higher in the keratoconus group at 0.35 (0.31, 0.52) µm than in the post-LASIK ectasia and PMD groups at 0.17 (0.12, 0.21) µm and 0.07 (0.02, 0.46) µm, respectively (p < 0.05). CONCLUSIONS: The KeraCare scleral contact lens reduced ocular aberrations and improved visual acuity in patients with post-LASIK ectasia, keratoconus and PMD. The sign of vertical coma changed in keratoconus and PMD.

Scleral Lens Visual Rehabilitation of Sequential Bilateral Corneal Hydrops With Post-LASIK Ectasia.

ABSTRACT: This case report describes a unique presentation of bilateral sequential acute corneal hydrops that manifested several years after laser in situ keratomileusis. Initial management included anterior chamber perfluoropropane gas injection and corneal suturing. Longer-term visual rehabilitation involved the use of scleral lenses which significantly reduced lower- and higher-order ocular aberrations.

Scleral lens wear following penetrating keratoplasty: changes in corneal curvature and optics.

PURPOSE: Visual rehabilitation following penetrating keratoplasty is the primary indication for approximately 15% of all scleral lens fittings. Since corneal biomechanics are altered following penetrating keratoplasty, the aim of this study was to quantify changes in anterior corneal optics following short-term scleral lens wear in eyes with corneal grafts. METHODS: Scheimpflug images were obtained before and after a period of scleral lens wear (mean 6.3 ± 1.4 h), from eyes that had previously undergone penetrating keratoplasty (10 eyes of nine participants, mean age 31 ± 9 years). Corneal power and thickness data were examined over the central 6 mm, including regional analyses of the central (0-3 mm) and the mid-peripheral cornea (3-6 mm annulus) using customised software to deterime corneal power vectors M (best fit sphere), J0 (90/180 astigmatism) and J45 (45/135 astigmatism). Anterior corneal aberrations were extracted using corneal elevation data. RESULTS: Corneal power vector J45 increased following lens wear (by 0.22 ± 0.05 D, p = 0.003) across the central 6 mm, while M displayed regional variations following lens wear indicating larger changes further from the corneal centre (p = 0.004). The change in corneal power vector M was also correlated with the magnitude of central corneal swelling (r = 0.65, p = 0.04). The anterior corneal aberration terms of oblique astigmatism, hoirzontal coma, and spherical aberration also varied following lens wear (all p ≤ 0.01). The mean change in the corneal spherocylinder derived from the elevation data following lens wear was +0.14/-0.54 × 44 for a 6 mm corneal diameter. CONCLUSIONS: Clinically significant alterations in anterior corneal topography and higher order aberrations were observed following short-term scleral lens wear in eyes that had undergone penetrating keratoplasty. Spherocylindrical changes were approximately double the magnitude and more oblique in orientation compared to previous reports of healthy eyes. Changes in corneal power vector M may be related to epithelial corneal oedema.

Microscopic origin of pressure-induced phase-transitions in urea: a detailed investigation through first principles calculations.

The potential crystal structures and properties of urea as a function of pressure were studied using ab initio based electronic structure calculations. The enthalpy-pressure behavior shows that urea undergoes a pressure induced structural phase transition from P4[combining macron]21m (phase I) → P212121 (phase III) at 0.66 GPa with a volume collapse of 4.83%, driven by softening of the acoustic mode along the Γ-X direction. Another phase transition from the P212121 → P21212 structure was identified at 3.09 GPa. The violation of Born stability criteria in the P212121 structure along with softening of the acoustic mode in the U-R direction was responsible for the pressure induced phase transition. Furthermore, the application of pressure led to the breaking and formation of N-HO bonds in the crystal structure of urea during the phase transition, i.e., the H-acceptor capacitance of the oxygen atoms was varied between phases I/IV and -III. Band structure calculations were performed using a hybrid functional (Heyd, Scuseria and Ernzerhof, HSE) which includes a part of exact Fock-exchange. The computed electronic band structure showed that the urea polymorphs are insulators with a direct band gap of 6.21, 6.85 and 6.99 eV for phase-I, -III and -IV, respectively, at selected pressures. We have also presented the dielectric functions (real (ε1(ω)) and imaginary (ε2(ω)) parts), refractive index and absorption coefficients to explore the optical characteristics of the urea phases. The geometric interpretation of intermolecular interactions were quantitatively visualized using Hirshfeld surface analysis. Our results provide a complete picture of various properties of urea polymorphs that lay the foundation for further understanding of structures and their applications.

Ti-fraction-induced electronic and magnetic transformations in titanium oxide films.

Titanium dioxide has been widely used in modern industrial applications, especially as an effective photocatalyst. Recently, freestanding TiO2 films with a markedly reduced bandgap of ∼1.8 eV have been synthesized, indicating that the dimension has a considerable influence on the bulk band gap (>∼3 eV) and enhances the adsorption range of visible light. Titanium oxide compounds have various stoichiometries and versatile properties. Therefore, it is very necessary to explore the electronic properties and functionalities of other titanium oxide films with different stoichiometries. Here, we combined structure searches with first-principle calculations to explore candidate Ti-O films with different stoichiometries. In addition to the experimentally synthesized TiO2 film, the structure searches identified three new energetically and dynamically stable Ti-O films with stoichiometries of Ti3O5, Ti3O2, and Ti2O. Calculations show that the Ti-O films undergo several interesting electronic transformations as the Ti fraction increases, namely, from a wide-gap semiconductor (TiO2, 3.2 eV) to a narrow-gap semiconductor (Ti3O5, 1.80 eV) and then to metals (Ti3O2 and Ti2O) due to the abundance of unpaired Ti_d electrons. In addition to the electronic transformations, we observed nonmagnetic (TiO2) to ferromagnetic (Ti3O5, Ti3O2, and Ti2O) transformations. Notably, the Ti3O5 film possesses both narrow-gap semiconductive and ferromagnetic properties, with a large magnetic moment of 2.0 µB per unit cell; therefore, this film has high potential for use in applications such as spintronic devices. The results highlight metal fraction-induced electronic and magnetic transformations in transition metal oxide films and provide an alternative route for the design of new, functional thin-film materials.

Correction: Topological behaviour of ternary non-symmorphic crystals KZnX (X = P, As, Sb) under pressure and strain: a first principles study.

Correction for 'Topological behaviour of ternary non-symmorphic crystals KZnX (X = P, As, Sb) under pressure and strain: a first principles study' by Atahar Parveen et al., Phys. Chem. Chem. Phys., 2018, 20, 5084-5102.

A comparative study of the structure, stability and energetic performance of 5,5'-bitetrazole-1,1'-diolate based energetic ionic salts: future high energy density materials.

Developing novel energetic materials of high detonation performance and low sensitivity is one of the primary objectives related to explosive research. By employing ab initio calculations, a series of energetic ionic salts based on 5,5'-bitetrazole-1,1'-diolate (BTO) were thoroughly investigated to understand the structure-property-performance interrelationship. The physicochemical and detonation characteristics of these energetic ionic salts including structural, electronic, vibrational and performance parameters (heats of formation, detonation pressures, and detonation velocities) were discussed in detail. The strong intermolecular hydrogen bonding environment between the BTO2- anion and various cations is mainly responsible for prominent detonation performance and enhanced molecular stability. Such strong intermolecular hydrogen bonds are observed in hydrazine and hydroxylammonium cations compared to other cations. To predict an accurate band gap, electronic band structures of the studied energetic ionic salts (EIS) were calculated using the HSE06 hybrid functional and they are found to be wide band gap insulators with a bandwidth ranging from 4.33-5.05 eV. Careful inspection of various EIS revealed that the hydroxylammonium and hydrazine cations produce the highest density relative to other cations when combined with the BTO anion. The detonation characteristics of BTO2- are computed using EXPLO5 code. In particular, HA-BTO and TKX-50 exhibit high detonation pressure (38.85 and 40.23 GPa) and detonation velocities (9.94 and 9.91 km s-1), superior to those of traditional nitrogen-rich energetic materials with moderate sensitivities. These results highlight the importance of hydrogen bonding interactions in designing energetic salts for next-generation explosives, propellants, and pyrotechnics.

Topological behaviour of ternary non-symmorphic crystals KZnX (X = P, As, Sb) under pressure and strain: a first principles study.

An ab initio study on the impact of hydrostatic pressure and strain on the electronic properties of an unexplored class of ternary Zintl phases KZnX (X = P, As, Sb) is reported. Density functional theory (DFT) based studies revealed that all the three materials are direct band gap semiconductors under ambient conditions. We have theoretically demonstrated that KZnX can be driven into different metallic phases under pressure. In contrast, by applying strain the compounds can be realized as topological insulators. This is confirmed by the observed non-trivial topological character in the electronic band structure of the present ternary systems. For the precise determination of low energy band topology, the Tran Blaha modified Becke-Johnson (TBmBJ) exchange potential was used by incorporating spin-orbit coupling. The concomitant change of electronic band shapes as a function of pressure indicates a semi-metallic nature in KZnX (X = P, As, Sb) at 30 GPa, 21 GPa and 11 GPa respectively. Based on an analysis of the parity eigenvalues, we anticipate that a band inversion occurs between the Zn-s and X-p states, thus demonstrating a weak topological behaviour in semi-metallic states. Also, a weak non-trivial topologically insulating phase is realized in strained KZnAs (18%) and KZnSb (10%) which appears to be due to overlapping of the Zn-s and X-p orbitals. The calculated surface spectral functions further validate the non-triviality of strained KZnX (X = As, Sb), whereas strained KZnP is found to be a trivial insulator. We confirm the topological behaviour of these materials by calculating topological surface states and defining a Z2 topological invariant. Our work based on sophisticated first-principles calculations highlights that both pressure and strain can trigger topological phases in non-symmorphic trivial band insulators even with a weak spin orbit interaction. This study paves the way for realizing semi-metallic and topological insulating states in non-symmorphic ternary semiconductors, which have not been experimentally demonstrated so far.

Role of spin-orbit interaction on the nonlinear optical response of CsPbCO3F using DFT.

We explore the effect of spin-orbit interaction (SOI) on the electronic and optical properties of CsPbCO3F using the full potential linear augmented plane wave method with the density functional theory (DFT) approach. CsPbCO3F is known for its high powder second harmonic generation (SHG) coefficient (13.4 times (d36 = 0.39 pm V-1) that of KH2PO4 (KDP)). Calculations are done for many exchange correlation (XC) potentials. After the inclusion of SOI, the calculated Tran-Blaha modified Becke-Johnson (TB-mBJ) band gap of 5.58 eV reduces to 4.45 eV in agreement with the experimental value. This is due to the splitting of Pb p-states. Importantly, the occurrence of a band gap along the H-A direction (indirect) transforms to the H-H (direct) high symmetry points/direction in the first Brillouin zone. We noticed a large anisotropy in the calculated complex dielectric function, absorption, and refractive index spectra. The calculated static birefringence of 0.1049 and 0.1057 (with SOI) is found to be higher than that of the other carbonate fluorides. From the Born effective charge (BEC) analysis we notice that the Cs atom shows a negative contribution to birefringence whereas Pb, C, and F atoms show a positive contribution. In addition, we have also calculated the nonlinear optical χ(-2ω;ω,ω) dispersion of a CsPbCO3F single crystal. We found that d11 = d12 = 4.35 pm V-1 at 1064 nm, which is 11.2 times higher than d36 of KDP. The origin of the highly nonlinear optical susceptibility dispersion of CsPbCO3F is explained. Overall, our results are in agreement with experiments and it is obvious from the present study that CsPbCO3F is a direct band gap, large second harmonic generation, and good phase matchable NLO crystal in the ultraviolet region.

ZnGeSb2: a promising thermoelectric material with tunable ultra-high conductivity.

First principles calculations predict the promising thermoelectric material ZnGeSb2 with a huge power factor (S2σ/τ) on the order of 3 × 1017 W m-1 K-2 s-1, due to the ultra-high electrical conductivity scaled by a relaxation time of around 8.5 × 1025 Ω-1 m-1 s-1, observed in its massive Dirac state. The observed electrical conductivity is higher than the well-established Dirac materials, and is almost carrier concentration independent with similar behaviour of both n and p type carriers, which may certainly attract device applications. The low range of thermal conductivity is also evident from the phonon dispersion. Our present study further reports the gradual phase change of ZnGeSb2 from a normal semiconducting state, through massive Dirac states, to a topological semi-metal. The maximum power factor is observed in the massive Dirac states compared to the other two states.

Phase stability and lattice dynamics of ammonium azide under hydrostatic compression.

We have investigated the effect of hydrostatic pressure and temperature on phase stability of hydro-nitrogen solids using dispersion corrected density functional theory calculations. From our total energy calculations, ammonium azide (AA) is found to be the thermodynamic ground state of N4H4 compounds in preference to trans-tetrazene (TTZ), hydro-nitrogen solid-1 (HNS-1) and HNS-2 phases. We have carried out a detailed study on structure and lattice dynamics of the equilibrium phase (AA). AA undergoes a phase transition to TTZ at around ∼39-43 GPa followed by TTZ to HNS-1 at around 80-90 GPa under the studied temperature range 0-650 K. The accelerated and decelerated compression of a and c lattice constants suggest that the ambient phase of AA transforms to a tetragonal phase and then to a low symmetry structure with less anisotropy upon further compression. We have noticed that the angle made by type-II azides with the c-axis shows a rapid decrease and reaches a minimum value at 12 GPa, and thereafter increases up to 50 GPa. Softening of the shear elastic moduli is suggestive of a mechanical instability of AA under high pressure. In addition, we have also performed density functional perturbation theory calculations to obtain the vibrational spectrum of AA at ambient as well as at high pressures. Furthermore, we have made a complete assignment of all the vibrational modes which is in good agreement with the experimental observations at ambient pressure. Moreover, the calculated pressure dependent IR spectra show that the N-H stretching frequencies undergo red and blue-shifts corresponding to strengthening and weakening of hydrogen bonding, respectively, below and above 4 GPa. The intensity of the N-H asymmetric stretching mode B2u is found to diminish gradually and the weak coupling between NH4 and N3 ions makes B1u and B3u modes to degenerate with progression of pressure up to 4 GPa which causes weakening of hydrogen bonding and these effects may lead to a structural phase transition in AA around 4 GPa. Furthermore, we have also calculated the phonon dispersion curves at 0 and 6 GPa and no soft phonon mode is observed under high pressure.

Dispersion Corrected Structural Properties and Quasiparticle Band Gaps of Several Organic Energetic Solids.

We have performed ab initio calculations for a series of energetic solids to explore their structural and electronic properties. To evaluate the ground state volume of these molecular solids, different dispersion correction methods were accounted in DFT, namely the Tkatchenko-Scheffler method (with and without self-consistent screening), Grimme's methods (D2, D3(BJ)), and the vdW-DF method. Our results reveal that dispersion correction methods are essential in understanding these complex structures with van der Waals interactions and hydrogen bonding. The calculated ground state volumes and bulk moduli show that the performance of each method is not unique, and therefore a careful examination is mandatory for interpreting theoretical predictions. This work also emphasizes the importance of quasiparticle calculations in predicting the band gap, which is obtained here with the GW approximation. We find that the obtained band gaps are ranging from 4 to 7 eV for the different compounds, indicating their insulating nature. In addition, we show the essential role of quasiparticle band structure calculations to correlate the gap with the energetic properties.

Structural stability, vibrational, and bonding properties of potassium 1, 1'-dinitroamino-5, 5'-bistetrazolate: An emerging green primary explosive.

Potassium 1,1'-dinitroamino-5,5'-bistetrazolate (K2DNABT) is a nitrogen rich (50.3% by weight, K2C2N12O4) green primary explosive with high performance characteristics, namely, velocity of detonation (D = 8.33 km/s), detonation pressure (P = 31.7 GPa), and fast initiating power to replace existing toxic primaries. In the present work, we report density functional theory (DFT) calculations on structural, equation of state, vibrational spectra, electronic structure, and absorption spectra of K2DNABT. We have discussed the influence of weak dispersive interactions on structural and vibrational properties through the DFT-D2 method. We find anisotropic compressibility behavior (b

Structural, electronic and optical properties of well-known primary explosive: Mercury fulminate.

Mercury Fulminate (MF) is one of the well-known primary explosives since 17th century and it has rendered invaluable service over many years. However, the correct molecular and crystal structures are determined recently after 300 years of its discovery. In the present study, we report pressure dependent structural, elastic, electronic and optical properties of MF. Non-local correction methods have been employed to capture the weak van der Waals interactions in layered and molecular energetic MF. Among the non-local correction methods tested, optB88-vdW method works well for the investigated compound. The obtained equilibrium bulk modulus reveals that MF is softer than the well known primary explosives Silver Fulminate (SF), silver azide and lead azide. MF exhibits anisotropic compressibility (b > a > c) under pressure, consequently the corresponding elastic moduli decrease in the following order: C22 > C11 > C33. The structural and mechanical properties suggest that MF is more sensitive to detonate along c-axis (similar to RDX) due to high compressibility of Hg⋯O non-bonded interactions along that axis. Electronic structure and optical properties were calculated including spin-orbit (SO) interactions using full potential linearized augmented plane wave method within recently developed Tran-Blaha modified Becke-Johnson (TB-mBJ) potential. The calculated TB-mBJ electronic structures of SF and MF show that these compounds are indirect bandgap insulators. Also, SO coupling is found to be more pronounced for 4d and 5d-states of Ag and Hg atoms of SF and MF, respectively. Partial density of states and electron charge density maps were used to describe the nature of chemical bonding. Ag-C bond is more directional than Hg-C bond which makes SF to be more unstable than MF. The effect of SO coupling on optical properties has also been studied and found to be significant for both (SF and MF) of the compounds.

Polymorphism and thermodynamic ground state of silver fulminate studied from van der Waals density functional calculations.

Silver fulminate (AgCNO) is a primary explosive, which exists in two polymorphic phases, namely, orthorhombic (Cmcm) and trigonal (R3) forms at ambient conditions. In the present study, we have investigated the effect of pressure and temperature on relative phase stability of the polymorphs using planewave pseudopotential approaches based on Density Functional Theory (DFT). van der Waals interactions play a significant role in predicting the phase stability and they can be effectively captured by semi-empirical dispersion correction methods in contrast to standard DFT functionals. Based on our total energy calculations using DFT-D2 method, the Cmcm structure is found to be the preferred thermodynamic equilibrium phase under studied pressure and temperature range. Hitherto Cmcm and R3 phases denoted as α- and ß-forms of AgCNO, respectively. Also a pressure induced polymorphic phase transition is seen using DFT functionals and the same was not observed with DFT-D2 method. The equation of state and compressibility of both polymorphic phases were investigated. Electronic structure and optical properties were calculated using full potential linearized augmented plane wave method within the Tran-Blaha modified Becke-Johnson potential. The calculated electronic structure shows that α, ß phases are indirect bandgap insulators with a bandgap values of 3.51 and 4.43 eV, respectively. The nature of chemical bonding is analyzed through the charge density plots and partial density of states. Optical anisotropy, electric-dipole transitions, and photo sensitivity to light of the polymorphs are analyzed from the calculated optical spectra. Overall, the present study provides an early indication to experimentalists to avoid the formation of unstable ß-form of AgCNO.

Structural, vibrational, and quasiparticle band structure of 1,1-diamino-2,2-dinitroethelene from ab initio calculations.

The effects of pressure on the structural and vibrational properties of the layered molecular crystal 1,1-diamino-2,2-dinitroethelene (FOX-7) are explored by first principles calculations. We observe significant changes in the calculated structural properties with different corrections for treating van der Waals interactions to Density Functional Theory (DFT), as compared with standard DFT functionals. In particular, the calculated ground state lattice parameters, volume and bulk modulus obtained with Grimme's scheme, are found to agree well with experiments. The calculated vibrational frequencies demonstrate the dependence of the intra and inter-molecular interactions on FOX-7 under pressure. In addition, we also found a significant increment in the N-H...O hydrogen bond strength under compression. This is explained by the change in bond lengths between nitrogen, hydrogen, and oxygen atoms, as well as calculated IR spectra under pressure. Finally, the computed band gap is about 2.3 eV with generalized gradient approximation, and is enhanced to 5.1 eV with the GW approximation, which reveals the importance of performing quasiparticle calculations in high energy density materials.

Experimental and theoretical investigation of Low-Frequency vibrational modes of 4-Amino 3,5 Dinitro Pyrazole in terahertz frequency domain.

Pyrazoles have recently received significant attention due to their unique and potential applications in the medical field, agriculture and are also known to be highly stable explosives. The present work describes the terahertz time-domain spectroscopy (THz-TDS) based study of 4-Amino 3,5 Dinitro Pyrazole(ADNP) in between the 0.1 and 3.0 THz ranges. A Toptica-Teraflash fibre-coupled handheld terahertz system has been employed in reflection mode configuration. We ascertained complex refractive index, absorption coefficients, and complex dielectric constants from 0.1 THz to 3.0 THz. The value of the refractive index and absorption coefficients are found to be 1.8 and 10---180 cm -1, respectively. Also, we have analyzed the structural, vibrational, and optical properties of ADNP using the plane-wave pseudopotential method based on Density Functional Theory (DFT) calculations. We have observed six low-frequency optical phonon modes, located at 0.36, 1.20, 1.52, 1.77, 2.40, and 2.75 THz, respectively, exhibiting a redshift compared to the values predicted by the DFT calculations. The possible reasoning for the above might be due to the anharmonicity that is not considered in the DFT calculations. The theoretical calculations align with the experimental results and deliver direction for further investigations and the futuristic application of ADNP.

Ferroelectric polymorphic phenomena in the layered antiferromagnet Cu(OH)2.

Ferroic orders and their associated structural phase transitions are paramount in the understanding of a multitude of unconventional condensed matter phenomena. On this note, our investigation focuses on the polymorphic ferroelectric (FE) phase transitions of Copper(II) hydroxide, Cu(OH)2, considering an antiferromagnetic ground state. By employing the first-principles studies and group theory analysis, we have provided a systematic theoretical investigation of vibrational properties in the hypotheticalCmcmhigh-symmetry phase to unveil the symmetry-allowed ferroic phases. We identified a non-polar to polar (Cmc21) phase transition, in which the displacive transformation is primarily responsible for the phase change induced by twoB1u(i.e.Γ2-) phonon modes within the centrosymmetric phase. We also observed the existence of two polar structures with the same space group and different degrees of polarization (i.e.Ps= 3.06µC·cm-2andPs= 42.41µC·cm-2), emerging from the high symmetry non-polar structure. According to the structural analysis the FE order, of a geometric nature, is driven by theΓ2-mode in which the O- and H-sites displacements lead the polar distortion with a minor contribution from the Cu-sites. Interestingly, the 3d9:Cu2+Jahn-Teller distortion coupled with the orientational shifts of O-H atoms enhances the polarization.