Proton ordering in (NH4)3H(SO4)2 at low-temperature phase transitions.

Single-crystal neutron diffraction was used to investigate the H-atom disorder in triammonium hydrogen disulfate (TAHS), (NH4)3H(SO4)2, below room temperature. Crystal structure analysis of the monoclinic phase III shows an increase of proton ordering with decreasing temperature in the (SO4)H(SO4) dimer. Moreover, the NH4(+) groups on a general position begin ordering in this phase. The monoclinic unit cell of TAHS-IV doubles in the b direction and a slight distortion of SO4(2-) and NH4(+) tetrahedra is observed. The order parameter introduced by Landau was determined for the second-order II/III and III/IV phase transitions from the intensities of the superstructure reflections. TAHS-V has a triclinic space group and the crystal structure seems to be completely ordered according to a structure analysis by single-crystal X-ray diffraction measurements. In addition, the decisive role of the dynamical disorder of different ammonium groups on successive phase transitions is discussed. Additional peaks were observed by X-ray powder diffraction measurements at ∼ 70 K on cooling, which refers to the V/VII phase transition. These additional peaks remained up to ∼ 85 K on heating. They were described with a doubling of the unit cell along all three principal crystallographic directions.

Disorder of (NH4)3H(SO4)2 in the high-temperature phase I.

The highly disordered crystal structure of triammonium hydrogen disulfate, (NH(4))(3)H(SO(4))(2), in the high-temperature phase I was studied using single-crystal neutron diffraction. It is known that the O atom involved in hydrogen bonding between neighbouring SO(4) tetrahedra is disordered and takes a split-atom position, building a two-dimensional hydrogen-bond network in the (001) plane. The H atoms in these SO(4)-H-SO(4) hydrogen bonds are disordered and hence refined with a split-atom model. Moreover, from the much larger anisotropic mean-square displacements of ammonium protons the NH(4)(+) groups were refined with a reasonable split-atom model, and their motional behaviour was also analysed by rigid-body treatment. Finally, careful consideration was given to show possible supplementary proton migration between the ammonium protons and those of the hydrogen bonds in this high-temperature phase.

Magnetic behaviour of synthetic Co(2)SiO(4).

Synthetic Co(2)SiO(4) crystallizes in the olivine structure (space group Pnma) with two crystallographically non-equivalent Co positions and shows antiferromagnetic ordering below 50 K. We have investigated the temperature variation of the Co(2)SiO(4) magnetic structure by means of non-polarized and polarized neutron diffraction for single crystals. Measurements with non-polarized neutrons were made at 2.5 K (below T(N)), whereas polarized neutron diffraction experiments were carried out at 70 and 150 K (above T(N)) in an external magnetic field of 7 T parallel to the b axis. Additional accurate non-polarized powder diffraction studies were performed in a broad temperature range from 5 to 500 K with small temperature increments. Detailed symmetry analysis of the Co(2)SiO(4) magnetic structure shows that it corresponds to the magnetic (Shubnikov) group Pnma, which allows the antiferromagnetic configuration (G(x), C(y), A(z)) for the 4a site with inversion symmetry 1 (Co1 position) and (0,C(y),0) for the 4c site with mirror symmetry m (Co2 position). The temperature dependence of the Co1 and Co2 magnetic moments obtained from neutron diffraction experiments was fitted in a modified molecular-field model. The polarized neutron study of the magnetization induced by an applied field shows a non-negligible amount of magnetic moment on the oxygen positions, indicating a delocalization of the magnetic moment from Co towards neighbouring O owing to superexchange coupling. The relative strength of the exchange interactions is discussed based on the non-polarized and polarized neutron data.

Dynamic proton disorder and the II-I structural phase transition in (NH(4))(3)H(SO(4))(2).

X-ray powder diffraction, differential scanning calorimetry (DSC)/thermogravimetry (TG) and single-crystal neutron diffraction methods were used to investigate triammonium hydrogen disulfate (NH(4))(3)H(SO(4))(2) (TAHS) in the temperature range between 293 and 493 K. The temperature-dependent X-ray powder diffraction measurements show a clear hysteresis of the I <-->II phase transition of TAHS with transition temperatures of T(up) = 412.9 (1) K on heating and of T(down) = 402.6 (1) K on cooling. From the existence of hysteresis and from the jump-like changes of the lattice parameters, the I <--> II phase transition of TAHS is considered to be first order. With DSC/TG measurements we confirmed that there is only one phase transition between 293 and 493 K. Through careful investigation on single crystals of TAHS using neutron diffraction, the correct space group (C2/c) of room-temperature TAHS-II phase was confirmed. Crystal structure analysis by single-crystal neutron diffraction showed a strongly elongated displacement ellipsoid of the proton which lies in the middle of the (SO(4))H(SO(4)) dimer with \bar 1 local symmetry. The protons of the NH(4) groups also show strongly enlarged anisotropic mean-square displacements. These findings are interpreted in terms of a characteristic proton disorder in the TAHS-II phase.

Structural behaviour of synthetic Co2SiO4 at low temperatures.

Synthetic Co2SiO4 has an olivine structure with isolated SiO4 groups (space group Pnma) and shows magnetic ordering below 50 K. Single-crystal neutron diffraction was applied to determine precise crystal structure parameters at low temperatures. No structural phase transition was revealed in the temperature range 2.5-300 K. Lattice parameters were determined by high-resolution X-ray powder diffraction between 15 and 300 K. There is a clear evidence of an anomalous thermal expansion related to the magnetic phase transition which can be attributed to magnetostriction.