Structural aspects of the 530 degrees C phase transition in LaBGeO5

High-resolution neutron powder diffractometry has been used to characterize the structural phase transition which occurs in LaBGeO5 at 530 degrees C. The change in lattice parameters of the trigonal unit cell as a function of temperature, observed in the range from ambient to 700 degrees C, shows that the transition from the polar space group P3(1) to the non-polar space group P3(1)21 is associated with a collapse of the a-dimension, whereas the c-dimension and the cell volume increase monotonically with small changes of slope at the transition temperature. The structure of the lower-temperature phase, previously determined at ambient temperature by single-crystal x-ray diffraction, has been refined at 20, 500 and 525 degrees C and the structure of the higher-temperature phase determined from data taken at 535, 560 and 630 degrees C. The thermal evolution of the atomic coordinates shows that the deciding role in the transition is provided by a complex movement of the B-O tetrahedron, in which the main component is a rotation around the 3(1) axis. The transition is continuous and results from structural changes which are displacive but also show order-disorder character. This conclusion is in accordance with the known thermal, optical and dielectric properties of this new multifunction (Laser, ferroelectric and non-linear optic) material. In the high-temperature modification, the dynamically disordered double helical chain of B-O tetrahedra has no polarity in the c-direction. This is the main structural reason for the absence of ferroelectric properties above 530 OC. The lower transition temperature of 140 degrees C in the related phase LaBSiO5 may be understood by comparing the structural aspects of its transition with those of LaBGeO5 using the Abrahams-Jamieson-Kurtz criteria. The effect of isomorphic substitution for Ge with the smaller Si atoms is equivalent to pressure being applied to the helical B chain and results in a chain which is more tilted in the silicate than in the germanate.