STRUCTURAL ASPECTS OF THE 530-DEGREES-C PHASE-TRANSITION IN LABGEO5

High-resolution neutron powder diffractometry has been used to charact erize the structural phase transition which occurs in LaBGeO5 at 530 d egrees C. The change in lattice parameters of the trigonal unit cell a s 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 temperatu re. The structure of the lower-temperature phase, previously determine d 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 degre es C. The thermal evolution of the atomic coordinates shows that the d eciding role in the transition is provided by a complex movement of th e B-O tetrahedron, in which the main component is a rotation around th e 3(1) axis. The transition is continuous and results from structural changes which are displacive but also show order-disorder character. T his conclusion is in accordance with the known thermal, optical and di electric properties of this new multifunction (Laser, ferroelectric an d non-linear optic) material. In the high-temperature modification, th e dynamically disordered double helical chain of B-O tetrahedra has no polarity in the c-direction. This is the main structural reason for t he absence of ferroelectric properties above 530 OC. The lower transit ion temperature of 140 degrees C in the related phase LaBSiO5 may be u nderstood by comparing the structural aspects of its transition with t hose of LaBGeO5 using the Abrahams-Jamieson-Kurtz criteria. The effect of isomorphic substitution for Ge with the smaller Si atoms is equiva lent 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.