Mechanistically Guided Materials Chemistry: Synthesis of Ternary Nitrides, CaZrN2 and CaHfN2.

Recent computational studies have predicted many new ternary nitrides, revealing synthetic opportunities in this underexplored phase space. However, synthesizing new ternary nitrides is difficult, in part because intermediate and product phases often have high cohesive energies that inhibit diffusion. Here, we report the synthesis of two new phases, calcium zirconium nitride (CaZrN2) and calcium hafnium nitride (CaHfN2), by solid state metathesis reactions between Ca3N2 and MCl4 (M = Zr, Hf). Although the reaction nominally proceeds to the target phases in a 1:1 ratio of the precursors via Ca3N2 + MCl4 → CaMN2 + 2 CaCl2, reactions prepared this way result in Ca-poor materials (CaxM2-xN2, x < 1). A small excess of Ca3N2 (ca. 20 mol %) is needed to yield stoichiometric CaMN2, as confirmed by high-resolution synchrotron powder X-ray diffraction. In situ synchrotron X-ray diffraction studies reveal that nominally stoichiometric reactions produce Zr3+ intermediates early in the reaction pathway, and the excess Ca3N2 is needed to reoxidize Zr3+ intermediates back to the Zr4+ oxidation state of CaZrN2. Analysis of computationally derived chemical potential diagrams rationalizes this synthetic approach and its contrast from the synthesis of MgZrN2. These findings additionally highlight the utility of in situ diffraction studies and computational thermochemistry to provide mechanistic guidance for synthesis.

Triple ionic-electronic conducting oxides for next-generation electrochemical devices.

Triple ionic-electronic conductors (TIECs) are materials that can simultaneously transport electronic species alongside two ionic species. The recent emergence of TIECs provides intriguing opportunities to maximize performance in a variety of electrochemical devices, including fuel cells, membrane reactors and electrolysis cells. However, the potential application of these nascent materials is limited by lack of fundamental knowledge of their transport properties and electrocatalytic activity. The goal of this Review is to summarize and analyse the current understanding of TIEC transport and electrochemistry in single-phase materials, including defect formation and conduction mechanisms. We particularly focus on the discovery criteria (for example, crystal structure and ion electronegativity), design principles (for example, cation and anion substitution chemistry) and operating conditions (for example, atmosphere) of materials that enable deliberate tuning of the conductivity of each charge carrier. Lastly, we identify important areas for further advances, including higher chemical stability, lower operating temperatures and discovery of n-type TIEC materials.

Minimization of Atomic Displacements as a Guiding Principle of the Martensitic Phase Transformation.

We present a unifying description for the martensitic transformation of steel that accounts for important experimentally observable features of the transformation, namely, the Neumann bands, the interfacial (habit) plane between the transformed and untransformed phases and their orientation relationship. It is obtained through a simple geometric minimization of the total distance traveled by all the atoms from the austenite (fcc or γ) phase to the martensite (bcc or α) phase, without the need for any explicit energy minimization. Our description unites previously proposed mechanisms but it does not rely on assumptions and experimental knowledge regarding the shear planes and directions, or external adjustable parameters. We show how the Kurdjumov-Sach orientation relationship between the two phases and the {225}_{γ} habit plane, which have both been extensively reported in experiments, naturally emerge from the distance minimization. We also propose an explanation for the occurrence of a different orientation relationship (Pitsch) in thin films.

Matching crystal structures atom-to-atom.

Finding an optimal match between two different crystal structures underpins many important materials science problems, including describing solid-solid phase transitions and developing models for interface and grain boundary structures. In this work, we formulate the matching of crystals as an optimization problem where the goal is to find the alignment and the atom-to-atom map that minimize a given cost function such as the Euclidean distance between the atoms. We construct an algorithm that directly solves this problem for large finite portions of the crystals and retrieves the periodicity of the match subsequently. We demonstrate its capacity to describe transformation pathways between known polymorphs and to reproduce experimentally realized structures of semi-coherent interfaces. Additionally, from our findings, we define a rigorous metric for measuring distances between crystal structures that can be used to properly quantify their geometric (Euclidean) closeness.

Modified split tendon transfer of posterior tibialis muscle in the treatment of spastic equinovarus foot deformity: long-term results and comparison with the standard procedure.

INTRODUCTION: Split tendon transfer of tibialis posterior (SPOTT) is a treatment option for the hindfoot varus deformity in patients with cerebral palsy (CP). The purpose of this study was to present the long-term results of the newly modified SPOTT procedure developed by our senior author and compare it with the standard SPOTT technique in equinovarus foot deformity due to CP. METHOD: Our retrospective cohort study included patients with spastic foot deformity due to CP treated with the standard or modified SPOTT technique. Patients' age at the time of the surgery was ≥ five years with follow-up period of at least four years. Surgical outcomes were evaluated using Kling's criteria during the patient's last follow-up visit. RESULTS: The analysis included 124 patients (146 feet), where 105 feet were treated by the standard SPOTT technique and 41 feet by the modified SPOTT technique. Patients' median age at the time of the surgery was 11 years. Patients were followed-up for a median period of eight years during which the modified SPOTT technique showed significantly better surgical outcomes compared with the standard group (excellent/good results in 38 feet, 92.7%, vs. 79 feet, 75.2%, p = 0.02). Two groups of patients did not significantly differ in GMFCS level, age at the time of the surgery, or patient gender. There was similar distribution in CP patterns in the standard and modified groups; spastic hemiplegia was the most prevalent form, followed by spastic diplegia and spastic paraplegia. Overall, better surgical success was achieved in patients with GMFCS levels I-III (100%, 94.8%, and 69.8%, respectively). SPOTT procedure failure was frequently noticed in patients with GMFCS level IV (90.9%). CONCLUSION: The modified SPOTT procedure demonstrated efficiency and safety in patients with equinovarus foot deformity due to CP during the long-term follow-up. Compared with the standard procedure, the newly modified SPOTT technique showed significantly better surgical outcome, irrespective of the patients' gender, age, initial GMFCS level, and CP type.

Sampling Polymorphs of Ionic Solids using Random Superlattices.

Polymorphism offers rich and virtually unexplored space for discovering novel functional materials. To harness this potential approaches capable of both exploring the space of polymorphs and assessing their realizability are needed. One such approach devised for partially ionic solids is presented. The structure prediction part is carried out by performing local density functional theory relaxations on a large set of random supperlattices (RSLs) with atoms distributed randomly over different planes in a way that favors cation-anion coordination. Applying the RSL sampling on MgO, ZnO, and SnO_{2} reveals that the resulting probability of occurrence of a given structure offers a measure of its realizability explaining fully the experimentally observed, metastable polymorphs in these three systems.

Methylammonium Bismuth Iodide as a Lead-Free, Stable Hybrid Organic-Inorganic Solar Absorber.

Methylammonium lead halide (MAPbX3 ) perovskites exhibit exceptional carrier transport properties. But their commercial deployment as solar absorbers is currently limited by their intrinsic instability in the presence of humidity and their lead content. Guided by our theoretical predictions, we explored the potential of methylammonium bismuth iodide (MBI) as a solar absorber through detailed materials characterization. We synthesized phase-pure MBI by solution and vapor processing. In contrast to MAPbX3, MBI is air stable, forming a surface layer that does not increase the recombination rate. We found that MBI luminesces at room temperature, with the vapor-processed films exhibiting superior photoluminescence (PL) decay times that are promising for photovoltaic applications. The thermodynamic, electronic, and structural features of MBI that are amenable to these properties are also present in other hybrid ternary bismuth halide compounds. Through MBI, we demonstrate a lead-free and stable alternative to MAPbX3 that has a similar electronic structure and nanosecond lifetimes.

Thermoelectricity in transition metal compounds: the role of spin disorder.

At room temperature and above, most magnetic materials adopt a spin-disordered (paramagnetic) state whose electronic properties can differ significantly from their low-temperature, spin-ordered counterparts. Yet computational searches for new functional materials usually assume some type of magnetic order. In the present work, we demonstrate a methodology to incorporate spin disorder in computational searches and predict the electronic properties of the paramagnetic phase. We implement this method in a high-throughput framework to assess the potential for thermoelectric performance of 1350 transition-metal sulfides and find that all magnetic systems we identify as promising in the spin-ordered ground state cease to be promising in the paramagnetic phase due to disorder-induced deterioration of the charge carrier transport properties. We also identify promising non-magnetic candidates that do not suffer from these spin disorder effects. In addition to identifying promising materials, our results offer insights into the apparent scarcity of magnetic systems among known thermoelectrics and highlight the importance of including spin disorder in computational searches.

Synthesis of a mixed-valent tin nitride and considerations of its possible crystal structures.

Recent advances in theoretical structure prediction methods and high-throughput computational techniques are revolutionizing experimental discovery of the thermodynamically stable inorganic materials. Metastable materials represent a new frontier for these studies, since even simple binary non-ground state compounds of common elements may be awaiting discovery. However, there are significant research challenges related to non-equilibrium thin film synthesis and crystal structure predictions, such as small strained crystals in the experimental samples and energy minimization based theoretical algorithms. Here, we report on experimental synthesis and characterization, as well as theoretical first-principles calculations of a previously unreported mixed-valent binary tin nitride. Thin film experiments indicate that this novel material is N-deficient SnN with tin in the mixed ii/iv valence state and a small low-symmetry unit cell. Theoretical calculations suggest that the most likely crystal structure has the space group 2 (SG2) related to the distorted delafossite (SG166), which is nearly 0.1 eV/atom above the ground state SnN polymorph. This observation is rationalized by the structural similarity of the SnN distorted delafossite to the chemically related Sn3N4 spinel compound, which provides a fresh scientific insight into the reasons for growth of polymorphs of metastable materials. In addition to reporting on the discovery of the simple binary SnN compound, this paper illustrates a possible way of combining a wide range of advanced characterization techniques with the first-principle property calculation methods, to elucidate the most likely crystal structure of the previously unreported metastable materials.

Recurrence of giant cell tumour of bone: role of p53, cyclin D1, ß-catenin and Ki67.

PURPOSE: To determine various clinical, radiographic, and pathological parameters which may indicate an increased risk of Giant cell tumour of bone (GCTB) recurrence after surgical therapy. METHODS: The study included a total of 164 GCTB samples; 118 (72 %) primary tumours, and 46 (28 %) recurrences; which were analyzed on immunohistochemistry for expression of Ki67, p53, cyclin D1, and ß-catenin. RESULTS: Among 13 analyzed clinical, radiological, and histological variables, which presented possible predictive factors for the incidence of GCTB relapse, univariate logistic regression (ULR) extract three highly statistically significant parameters: 1) lesion localization, 2) nuclear p53 expression in mononuclear cells, and 3) nuclear cyclin D1 expression in giant multinuclear cells. The multivariate logistic regression (MLR), revealing that p53 expression in mononuclear cells was the most significant predictive factor (HR = 6,181 p < 0,001), the positivity of which indicated six times higher probability for recurrence in GCTB. The expression of cyclin D1 in giant cells, containing less than 15 nuclei, was also statistically significant (HR = 8,398, p = 0,038) for predicting the recurrence, and demonstrated eight times more frequent recurrence in positive tumours. CONCLUSIONS: This study confirmed independent predicting factors for GCTB reccurence: p53 expression in mononuclear tumour cells and cyclin D1 expression in giant multinuclear cells. Results are new addition to generally known parameters, such as: localization of lesion, number of surgical interventions, clear destruction of cortex with the presence of extracompartmental lesion, and histological criteria for malignancy and can help in further research and treatment of GCTB.

Infrared assessment of knee instability in ACL deficient patients.

PURPOSE: Previous clinical studies have shown that anterior cruciate ligament (ACL) ruptures require reconstructive surgery. The main goal of this study is an objective test definition for unstable knee diagnosis based on real measurements by using infrared cameras and adequate software. METHODS: In the study of gait analysis 35 males with deficient ACL's participated. Pathological parameters for anterior posterior translation (APT) and internal external rotation (IER) and their values of kinematic data were obtained from a gait analysis 3D system. Movement curves were obtained by recording the position of fluorescent markers over time. A machine learning algorithm was developed in order to support decisions on the severity of the ACL injury and its corresponding deficiency. The algorithm was based on logistic regression. RESULTS: The value of APT, designated as exponentiation of the Ó¨ coefficient (Exp (Ó¨)) of APT, showed that the likelihood of ACL-deficient knee occurrence due to higher values of APT is 1.1758 (95 % CI) times more frequent than that of the patients with lower values of APT. The value of IER, designated as Exp (Ó¨) of IER, showed that the patients with higher values of IER present 2.2516 (95 % CI) times higher values of ACL-deficient knee frequency than those with lower values. CONCLUSION: This study showed that the creation of ordered pairs of pathological parameters gives a wider picture of ACL deficiency and that such an algorithm may improve both examination and treatment of patients.

Microsurgical anatomy of the extra-articular segment of middle genicular artery.

PURPOSE: Knowledge of the incompletely studied microsurgical anatomy of the extracapsular part of the middle genicular artery (MGA) could imply an educational value and clinical significance because of the possible risk of injury during knee surgery. METHODS: Thirty formol-fixed cadaveric lower limbs in full extension were dissected and used for the measurements of MGA parameters. A second group of measurements was performed on distal ends of 30 adult femurs. Two fresh injected cadaveric lower limbs were explored by means of multidetector computed tomographic angiography (MDCTA). RESULTS: The MGA originated from the popliteal artery (PA), facing the lateral half of the intercondylar fossa in 16 (53.4 %) specimens, together with the superior lateral genicular artery (SLGA) in ten (33.3 %) cases, or from the same point of origin with SLGA and superior medial genicular artery (SMGA) in 4 (13.3 %) cases. The MGA averaged 15.6 mm in length and 1.8 mm in the outer diameter. After its curved direction the MGA entered the posterior capsule. The average distances of the point of MGA entrance into the joint capsule were as follows: to the lateral femoral epicondyle it was 34.88 mm, to the medial femoral epicondyle 46.38 mm, 5.74 mm lateral to the posterior midline, with an average vertical distance to the femoral subcondylar plane of 28.73 mm. CONCLUSION: This detailed anatomical examination with measurements of the extracapsular part of a MGA could be of clinical importance and useful in knee surgery for the prevention of vascular injury of MGA and PA, as well as in radiological examination of the knee region.

Assessing capability of semiconductors to split water using ionization potentials and electron affinities only.

We show in this article that the position of semiconductor band edges relative to the water reduction and oxidation levels can be reliably predicted from the ionization potentials (IP) and electron affinities (AE) only. Using a set of 17 materials, including transition metal compounds, we show that accurate surface dependent IPs and EAs of semiconductors can be computed by combining density functional theory and many-body GW calculations. From the extensive comparison of calculated IPs and EAs with available experimental data, both from photoemission and electrochemical measurements, we show that it is possible to sort candidate materials solely from IPs and EAs thereby eliminating explicit treatment of semiconductor/water interfaces. We find that at pH values corresponding to the point of zero charge there is on average a 0.5 eV shift of IPs and EAs closer to the vacuum due to the dipoles formed at material/water interfaces.

Substrate-controlled band positions in CH3NH3PbI3 perovskite films.

Using X-ray and ultraviolet photoelectron spectroscopy, the surface band positions of solution-processed CH3NH3PbI3 perovskite thin films deposited on an insulating substrate (Al2O3), various n-type (TiO2, ZrO2, ZnO, and F:SnO2 (FTO)) substrates, and various p-type (PEDOT:PSS, NiO, and Cu2O) substrates are studied. Many-body GW calculations of the valence band density of states, with spin-orbit interactions included, show a clear correspondence with our experimental spectra and are used to confirm our assignment of the valence band maximum. These surface-sensitive photoelectron spectroscopy measurements result in shifting of the CH3NH3PbI3 valence band position relative to the Fermi energy as a function of substrate type, where the valence band to Fermi energy difference reflects the substrate type (insulating-, n-, or p-type). Specifically, the insulating- and n-type substrates increase the CH3NH3PbI3 valence band to Fermi energy difference to the extent of pinning the conduction band to the Fermi level; whereas, the p-type substrates decrease the valence band to Fermi energy difference. This observation implies that the substrate's properties enable control over the band alignment of CH3NH3PbI3 perovskite thin-film devices, potentially allowing for new device architectures as well as more efficient devices.

Semitendinosus tendon regeneration after anterior cruciate ligament reconstruction: can we use it twice?

PURPOSE: It has been demonstrated that the semitendinosus tendon can regenerate after being harvested in its whole length and thickness for anterior cruciate ligament (ACL) reconstruction. Ultrasound studies and guided biopsies of the regenerated tendon have shown compatibility and resembling features of the normal tendon. The question is if this neo-tendon is biologically and functionally adequate for re-use? METHODS: Two randomised groups of 150 volunteers were followed up for two years after harvesting the semitendinosus only (25) or the semitendinosus and gracilis tendons (25) in ACL reconstruction. The patients were followed up with clinical and ultrasound examinations, biopsies and histological tests. Surgical exploration was done in three patients for macroscopic verification. The injected arteries of four lower limbs were dissected and the tendon's arterial supplies were examined. RESULTS: Seventy-two percent of the cases showed regeneration of the semitendinosus tendons. The neotendons were inserted mostly below the knee joint (83.3%) where they had fused with the gracilis tendon, and above the joint (60%) when the gracilis was harvested as well. The isokinetic strength of the hamstrings and quadriceps was not significantly diminished on the operated side. A macroscopic and histological analysis of the regenerated tendons demonstrates close resemblance to normal anatomy, with focal areas of fibrosis. In one patient the regenerated tendon was used for medial patellofemoral ligament reconstruction. CONCLUSION: The semitendinosus muscle can recover and the tendon has great potential to regenerate after harvesting for ACL reconstruction. Our data suggest that the regenerated tendons could be used for iterative ligament reconstruction.

A search for new back contacts for CdTe solar cells.

There is widespread interest in reaching the practical efficiency of cadmium telluride (CdTe) thin-film solar cells, which suffer from open-circuit voltage loss due to high surface recombination velocity and Schottky barrier at the back contact. Here, we focus on back contacts in the superstrate configuration with the goal of finding new materials that can provide improved passivation, electron reflection, and hole transport properties compared to the commonly used material, ZnTe. We performed a computational search among 229 binary and ternary tetrahedrally bonded structures using first-principles methods and transport models to evaluate critical material design criteria, including phase stability, electronic structure, hole transport, band alignments, and p-type dopability. Through this search, we have identified several candidate materials and their alloys (AlAs, AgAlTe2, ZnGeP2, ZnSiAs2, and CuAlTe2) that exhibit promising properties for back contacts. We hope that these new material recommendations and associated guidelines will inspire new directions in hole transport layer design for CdTe solar cells.

Correction: The importance of phase equilibrium for doping efficiency: iodine doped PbTe.

Correction for 'The importance of phase equilibrium for doping efficiency: iodine doped PbTe' by James Male et al., Mater. Horiz., 2019, 6, 1444-1453, DOI: 10.1039/C9MH00294D.

Universal electrostatic origin of cation ordering in A2BO4 spinel oxides.

The crystal structures of A(2)BO(4) spinel oxides are classified as either normal or inverse, representing different distributions of the A and B cations over the tetrahedrally and octahedrally coordinated cation sites. These structures undergo characteristic structural changes as a function of temperature: (i) the nominally disordered inverse structure orders crystallographically at low T, and (ii) at finite temperatures, both inverse and normal develop characteristic distributions of cations associated with order-disorder structural changes. We show here that all of these universal features emerge naturally from a simple point-ion electrostatic (PIE) model with a single adjustable parameter. Monte Carlo simulations of the PIE Hamiltonian provide quantitative order-disorder characteristic temperatures. We show that, with the help of the PIE model, the magnitude of the temperatures can be inferred from the nominal charges of the atomic species in the spinel. Indeed, we show that characteristic order-disorder temperatures in 3-2 spinels (nominal charges Z(A) = 3 and Z(B) = 2) are approximately an order of magnitude lower than in 2-4 spinels, thus explaining why typical 3-2 samples exhibit much larger degrees of disorder than those belonging to the 2-4 class.

Predicting energy and stability of known and hypothetical crystals using graph neural network.

The discovery of new inorganic materials in unexplored chemical spaces necessitates calculating total energy quickly and with sufficient accuracy. Machine learning models that provide such a capability for both ground-state (GS) and higher-energy structures would be instrumental in accelerated screening. Here, we demonstrate the importance of a balanced training dataset of GS and higher-energy structures to accurately predict total energies using a generic graph neural network architecture. Using ∼ 16,500 density functional theory calculations from the National Renewable Energy Laboratory (NREL) Materials Database and ∼ 11,000 calculations for hypothetical structures as our training database, we demonstrate that our model satisfactorily ranks the structures in the correct order of total energies for a given composition. Furthermore, we present a thorough error analysis to explain failure modes of the model, including both prediction outliers and occasional inconsistencies in the training data. By examining intermediate layers of the model, we analyze how the model represents learned structures and properties.

Simple point-ion electrostatic model explains the cation distribution in spinel oxides.

The A2BO4 spinel oxides are distinguished by having either a normal (N) or an inverse (I) distribution of the A, B cations on their sublattices. A point-ion electrostatic model parametrized by the oxygen displacement parameter u and by the relative cation valencies Z{A} vs Z{B} provides a simple rule for the structural preference for N or I: if Z{A}>Z{B} the structure is normal for u>0.2592 and inverse for u<0.2578, while if Z{A}0.2578. This rule is illustrated for the known spinel oxides, proving to be ∼98% successful.