Andrzej Grzechnik

Crystal structures and stability of NaLnF4 (Ln = La, Ce, Pr, Nd, Sm and Gd) studied with synchrotron single-crystal and powder diffraction

Crystal structures of the NaLnF(4) materials (Ln = La, Ce, Pr, Nd, Sm or Gd) were studied with synchrotron single-crystal and powder diffraction. The materials with Ln = La, Ce, Nd, Sm and Gd have the average β structure (P6, Z = 1) with partial ordering of the cations. A new type of a superstructure due to ordering of the cations and vacancies was found in NaPrF(4) (P3, Z = 6). It could be described using the group-subgroup relationships P6↔P3. Our observations suggest that the β structure is unstable and that the ordering is a slow process at ambient conditions. Upon compression, β-NaNdF(4), β-NaGdF(4) and the superstructure NaPrF(4) are stable to at least 8 GPa with no evidence for any pressure-induced disorder-order phenomena.

Twinning and pseudosymmetry under high pressure

Abstract We review the available information on crystal twinning at high pressures and analyze the effect of hydrostatic pressure on the pseudosymmetry of selected compounds. Twinning by merohedry, type I and II, as well as pseuomerohedral twinning at high pressures has been described. For twinning by merohedry type I (inversion twinning) a reliable characterization of the twin domains and volume fractions is difficult and largely depends on the experimental conditions, i.e. the number of measured Friedel pairs and the chosen wavelength. For twinning by merohedry, type II, and for twinning by pseuodmerohedry twin volume fractions could be reliably determined for several cases. In none of these a significant influence of hydrostatic pressure on the volume fractions of the individuals was observed. Pressure-induced twinning has been observed for compounds which undergo first-order phase transitions. The twinning operation in the described cases is related to the loss of rotational symmetry elements of the higher symmetrical polymorph although the high and low pressure phases are not in group-subgroup relationship. The analysis of pseudosymmetry of several compounds as a function of pressure suggests that this parameter can be used to predict the (in)stability of compounds. In particular, a decrease in pseudosymmetry seems to be strongly correlated with the occurrence of first-order phase transitions in which the crystal breaks or amorphizes. The effect of hydrostatic and chemical pressure on the pseudosymmetry of the crystal structures can lead to different or even opposite trends, suggesting that the increase or decrease of pseudosymmetry cannot be understood as a pure volume effect.

Twinned caesium cerium(IV) penta-fluoride.

Single-crystals of CsCeF5 were synthesized hydrothermally. The crystal under investigation was twinned by pseudo-merohedry with a twofold rotation around the c axis as an additional twinning operation. The crystal structure is built of layers of distorted edge- and corner-sharing CeF8 square-antiprisms. The Cs+ cations are located between the layers and exhibit coordination numbers of nine. Upon compression, CsCeF5 undergoes an irreversible phase transition at about 1 GPa.