Phase transitions of modulated Co-åkermanite, Ca2CoSi2O7
Abstract
This paper deals with X-ray single crystal experiments of modulated cobalt åkermanite at temperatures between 30 K and 600 K. The primary objective of these investigations was to study the predicted lock-in phase transition into a commensurate low temperature phase. Upon cooling the phase transition occurs at 160 K, provided that the time for thermodynamic equilibration at each temperature step is guaranteed. The low temperature phase turns out to be incommensurate with a modulation wavelength of 3.086(6) · a0. This is very close to the predicted commensurate value of 3 · a0 The standard deviations of the q-values are small enough to determine this low temperature phase as significantly incommensurate. A fourier synthesis of the complementary structure (as a 3-fold superstructure) reveals a tetragonal displacement pattern of the cobalt-atoms at low temperatures. The second order phase transition to the parent structure occurs at 498(1) K. It is characterized by a critical exponent β = 0.31(1)°.
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