Modulated structure and phase transitions of Sr10Ga6O19.

ABSTRACT

The crystal structure of Sr10Ga6O19 was investigated by in situ single-crystal X-ray diffraction in the temperature range 298-673 K. At ambient conditions the compound shows a (3 + 1)-dimensional modulated structure in the superspace group C2/c(0beta0)s0 [a = 34.9145 (13), b = 7.9369 (2), c = 15.9150 (7) A and beta = 103.551 (3) degrees] with a modulation wavevector of q = 0.4288 (2)b*. Whereas the presented structural model uses first-order harmonic modulation functions only, some features of the modulations are discussed utilizing an electron density derived by the maximum entropy method. Furthermore, two phase transitions were identified between 453 and 503 K the incommensurate superstructure is replaced by a doubling of the a and b lattice constants, and between 503 and 673 K a phase with the basic cell is formed, identical to alpha-Sr10Ga6O19. Under some cooling conditions crystals showing a combined diffraction pattern of both superstructures can be obtained. The relation of these results to alpha-Sr10Ga6O19 [Kahlenberg (2001). J. Solid State Chem. 160, 421-429] is discussed.