B. V. Mill

Langasite-type materials: from discovery to present state

A review of the results on synthesis, crystal chemistry, single crystal growth of langasite family compounds up to the present day is presented together with a discussion of the main electromechanical parameters of the materials and structure-property correlations.

Accurate crystal-structure refinement of Ca3Ga2Ge4O14 at 295 and 100 K and analysis of the disorder in the atomic positions

The accurate X-ray diffraction study of a Ca3Ga2Ge4O14 crystal (sp. gr. P321, Z = 1) has been performed using repeated X-ray diffraction data sets collected on a diffractometer equipped with a CCD area detector at 295 and 100 K. The asymmetric disorder in the atomic positions in Ca3Ga2Ge4O14 is described in two alternative ways: with the use of anharmonic atomic displacements (at 295 K R/wR = 0.68/0.60%, 3754 reflections; at 100 K R/wR = 0.90/0.70%, 3632 reflections) and using a split model (SM) (at 295 K R/wR = 0.74/0.67%; at 100 K R/wR = 0.95/0.74%). An analysis of the probability density function that defines the probability of finding an atom at a particular point in space shows that, at 295 K, five of the seven independent atoms in the unit cell are asymmetrically disordered in the vicinity of their sites, whereas only three atoms are disordered at 100 K. At both temperatures the largest disorder is observed at the 3f site on a twofold axis, which is a prerequisite for the formation of helicoidal chains of atoms along the c axis of the crystal and can serve as a structural basis for multiferroic properties of this family of crystals with magnetic ions.

Refinement of the crystal structures of the La3Ta0.5Ga5.5O14 and La3Nb0.5Ga5.5O14 compounds

This paper reports on the results of accurate X-ray structural investigations of single crystals La3Ta0.5Ga5.5O14 (a = 8.2260(1) Å, c = 5.1207(1) Å, R/Rw = 1.09%/1.10%, 3868 unique reflections) and La3Nb0.5Ga5.5O14 (a = 8.2237(1) Å, c = 5.1247(1) Å, R/Rw = 1.02%/1.03%, 3735 unique reflections) (space group P321, Z = 1, sinϑ/λ|max ≈ 1.34 Å−1 for both compounds). Using sets of data with a large number of high-angle reflections makes it possible to reliably compare the crystal structures by applying statistical tests. The structural differences between the La3Ta0.5Ga5.5O14 and La3Nb0.5Ga5.5O14 crystals are weakly pronounced, which correlates with the closeness of their piezoelectric characteristics.

X-ray diffraction study of a Nd3Ga5SiO14 crystal at 295 and 90 k and the structural basis for chirality

An accurate X-ray diffraction study of a Nd3Ga5SiO14 crystal (sp. gr. P321, Z = 1) has been performed using repeated X-ray diffraction data sets collected on a diffractometer equipped with a CCD area detector at 295 and 90 K. The asymmetry of the disorder in the atomic positions in Nd3Ga5SiO14 is described in two alternative ways: in terms of anharmonic atomic displacements (at 295 K R/wR = 0.80/0.92%; at 90 K R/wR = 0.82/0.76%) and using a split model (at 295 K R/wR = 0.85/0.97%; at 90 K R/wR = 0.80/0.74%). An analysis of the probability density function of finding an atom at a particular point in space shows that most of the atoms in the unit cell are asymmetrically or highly anisotropically disordered in the vicinity of their sites. Electron density regions for cations and anions that form pseudohelices were found. Such regions mimic a screw axis of symmetry and are responsible for the chirality of the structure.

Phase transitions in compounds with the Ca3Ga2Ge4O14 structure

Possible structural changes described by the group-subgroup relationships in the Ca3Ga2Ge4O14-type structure (sp. gr. P321) are considered. The most probable phase transitions seem to be those accompanied by lowering of the symmetry to the maximal non-isomorphic subgroups P3 and C2. It is shown that only destructive phase transitions accompanied by symmetry rise up to the minimal non-isomorphic supergroups for the given structure type can take place. The change of the trigonal symmetry to monoclinic is revealed in La3SbZn3Ge2O14, whose crystal structure is refined as a derivative structure of the Ca3Ga2Ge4O14 structure type within the sp. gr. A2 (C2). At ∼250°C, La3SbZn3Ge2O14 undergoes a reversible phase transition accompanied by symmetry rise, A2 ⇄ P321. Similar phase transitions, P321 ⇄ A2, are also observed in La3Nb0.5Ga5.5O14 and La3Ta0.5Ga5.5O14 under the hydrostatic pressures 12.4(3) and 11.7(3) GPa, respectively. The mechanisms of compression and phase transition are based on the anisotropic compressibility of a layer structure. With the attainment of the critical stress level in the structure, the elevated compressibility in the (ab) plane gives rise to a phase transition accompanied by the loss of the threefold axis. Attempts to reveal low-temperature phase transitions in a number of representatives of the langasite family have failed.

Crystal structure refinement of Sr3TaGa3Si2O14

An accurate X-ray diffraction study of Sr3TaGa3Si2O14 (STGS) crystal (a = 8.3023(10) Å, c = 5.0853(2) Å, sp. gr. P321, Z = 1, R/wR = 0.59/0.52%, 4004 independent reflections) is performed. The use of two independent data sets obtained on diffractometers with point and 2D detectors made it possible to determine the model structure characterized by the best reproducibility of parameters. The ordered distribution of atoms over crystallographic positions and the anharmonic character of displacements of all cations and one oxygen atom are established.

Enantiomorphism of the Ca3Ga2Ge4O14 compound and comparison of the Ca3Ga2Ge4O14 and Sr3Ga2Ge4O14 structures

The absolute crystal structures of two enantiomorphic forms of the Ca3Ga2Ge4O14 crystals (a = 8.075(1) Å, c = 4.9723(6) Å, space group P321) with the positive and negative senses of the optical activity are determined using X-ray diffraction analysis. The final R factors are as follows: R = 1.75% and Rw = 2.57% for the crystal with the positive sense of the optical activity and R = 1.86% and Rw = 2.78% for the crystal with the negative sense of the optical activity. The replacement of the Ca2+ ions by larger Sr2+ ions (with the formation of the Sr3Ga2Ge4O14 compound) leads to an anisotropic expansion of the crystal lattice (with a more considerable increase in the lattice parameter a as compared to the lattice parameter c), a change in the occupation of the 1a and 3f positions by the Ga3+ and Ge4+ ions, and symmetrization of the octahedra and tetrahedra forming the structural framework. The shape of the dodecahedron changes so that its size along the polar electric axis 2 increases significantly. This change is the main factor responsible for the increase in the piezoelectric activity of the Sr3Ga2Ge4O14 compound as compared to the piezoelectric activity of the Ca3Ga2Ge4O14 compound.

New ferroelectrics in KTiOPO4 family

Abstract New compounds AMOXO4 (A = Na, K, Rb, Tl; M = Sb, Ta; X = Ge, Si) were prepared by solid state synthesis and as single crystals by the flux method. Their crystal structure and ferroelectric properties are similar to those of KTiOPO4, but optical nonlinearity is much lower. For TlSbOGeO4 an antiferroelectricity was supposed to exist in the interval between two phase transitions at Tc = 272 K and TN = 286 K. According to values of Curie-Weiss constant (C = 5·103) the phase transitions in TlSbOGeO4 have features of order-disorder transformation while in KTaOGeO4 the value C = 1.2·105 corresponds to displacive one. Special role of the monovalent cations in KTiOPO4-type compounds is discussed taking into consideration data on ferroelectric, nonlinear optical properties and structure.

Circular dichroism spectra of langasite family crystals in the range of electronic transitions of structure defects

The absorption and circular dichroism spectra of langasite family crystals are studied. Wide bands in the range of 285–500 nm that are related to the structure lattice defects are found. For all the crystals, these bands are in approximately the same spectral region, have identical structures, and can be attributed to one type of defects formed by cation vacancies and excess oxygen atoms in the optically active positions.

The absorption and circular dichroism spectra of langasite family crystals doped with chromium ions

The absorption and circular dichroism spectra of langasite family crystals, La3Ga5SiO14, La3Ga5GeO14, Ca3Ga2Ge4O14, Sr3Ga2Ge4O14 (red), Sr3Ga2Ge4O14 (green), La3Ta0.5Ga5.5O14, and La3Nb0.5Ga5.5O14, which were doped with chromium ions, have been investigated in the range of 240–850 nm. It is shown that chromium ions are incorporated into the structure of the investigated crystals both in the octahedrally (Cr3+ ion in 1a octahedron) and tetrahedrally (Cr4+ ion in 2d tetrahedron) coordinated positions. The ion ratio Cr3+/Cr4+ changes in a wide range in the crystals studied.