Crystal growth and polymorphism of Li2MgSiO4

A method for growing Cr4+: Li2MgSiO4 single crystals with the longest lumin escence lifetime of all the orthosilicates and germanates is developed. The thermographic and X-ray diffraction analyses of the crystals grown showed that Li2MgSiO4 crystals undergo a reversible phase transition in the temper ature range from 498 to 618 degreesC. The low-temperature phase with the un it-cell parameters a = 6.2915(7), b = 10.688(1), c = 4.9946(5) Angstrom, be ta = 90.479(9)degrees, Z = 4 is a monoclinically distorted high-temperature modification (isostructural) to orthorhombic Li2MgGeO4 crystals. Within th e experimental error, the unit-cell parameters of the low-temperature phase linearly depend on temperature, but the angle beta varies from 90.38 degre es to 90 degrees. The coefficients of linear thermal expansion along the cr ystallographic axes are equal to 1.76 x 10(-5), 1.26 x 10(-5), and 1.74 x 1 0(-5) K-1; the coefficient of volume expansion is equal to 4.91 x 10(-5) K- 1. The phase transition is not accompanied by any essential transformation of the structural motif and causes no fracturing or defect formation. (C) 2 000 MAIK "Nauka/Interperiodica".