Volume 229, Issue 3

No Abstract

This paper presents a review of techniques and considerations in the design and construction of high pressure, low temperature diffraction experiments. Also intended as an introductory text to new high pressure users, the crucial aspects of pressure cell design are covered. The general classification of common designs, and a discussion into the key beam interaction, mechanical, and thermal properties of commonly used materials is giv­en. The advantages of different materials and high pressure cell classifications are discussed, and examples of designs developed for low temperature diffraction stud­ies are presented, and compared.

[Ni(en) 3 ][NO 3 ] 2 undergoes a displacive phase transition from P 6 3 22 at ambient pressure to a lower symmetry P 6 1 22/ P 6 5 22 structure between 0.82 and 0.87 GPa, which is characterized by a tripling of the unit cell c -axis and the number of molecules per unit cell. The same transition has been previously observed at 108 K. The ­application of pressure leads to a general shortening of O … H hydrogen bonding interactions in the structure, with the greatest contraction (24%) occurring diagonally between stacks of Ni cation moieties and nitrate anions.

Electronic/magnetic transitions and their structural consequences in Fe-based Mott insulators in a regime of very high static density are the main issue of this short review paper. The paper focuses on the above-mentioned topics based primarily on our previous and ongoing experimental HP studies employing: (i) diamond anvil cells, (ii) synchrotron X-ray diffraction, (iii) 57 Fe Mössbauer spectroscopy, (iv) electrical resistance and (v) X-ray absorption spectroscopy. It is shown that applying pressure to such strongly correlated systems leads to a number of changes; including quenching of the orbital moment, quenching of Jahn-Teller distortion, spin crossover, inter-valence charge transfer, insulator–metal transition, moment collapse and volume collapse. These changes may occur simultaneously or sequentially over a range of pressures. Any of these may be accompanied by or be a consequence of a structural phase transition; namely, a change in crystal symmetry. Analyzing this rich variety of phenomena we show the main scenarios which such strongly correlated systems may undergo on the way to a correlation breakdown (Mott transition). To illustrate these scenarios we present recent results for M FeO 3 ( M = Fe, Ga, Lu, Eu, Pr) and CaFe 2 O 4 ferric oxides; FeCl 2 and FeI 2 ferrous halides, and FeCr 2 S 4 sulfide. Fe 3 O 4 is given as an example case for the impact of Mössbauer Spectroscopy on High Pressure Crystallography studies.

Diffuse and Bragg scattering of synchrotron radiation on single crystal PMN relaxor has been collected as a function of pressure. The relaxor-specic diffuse scattering (DS) is observed for p < 40 kbar and successfully parametrized with the thermal DS-like model of polar correlations. An increase of pressure does suppress the polar correlations in a specic glass-like relaxor state, and above ~40 kbar a new non-polar R -phase becomes stable. We propose a phenomenological model accounting for off-center displacements of lead ions; together with previous reports these findings map the relaxor state to a specifc area of phase diagram.

The Zn 6 Sc 1/1 cubic approximant to a quasicrystal has been studied in-situ at high pressures by ­single-crystal X-ray diffraction. This phase can be described as a bcc packing of Tsai-type icosahedral clusters whose center is occupied by a disordered Zn 4 tetrahedron. At ambient pressure the Zn 6 Sc undergoes a structural phase transition at 159 K to a monoclinic superstructure in which the Zn 4 tetrahedra are orientationally ordered along the [1̄01] direction of the high-temperature bcc phase. In the pressure range up to 35 GPa, two new superstructures have been observed. The second phase corresponds to a four-fold pseudo cubic superstructure, i.e. a very large unit cell with a lattice parameter of about 5.5 nm. The resulting pressure-temperature phase diagram is different from that of Cd 6 Yb, which was reported by Watanuki et al. (2006).

High-pressure research of the crystals of or­ganic and coordination compounds is now a well-established field. However, the studies of multicomponent crystals (hydrates, solvates, salts and co-crystals) are just emerging. The aim of this paper is to give a personal view of what makes the multicomponent organic systems peculiar for high-pressure research as compared with the monocomponent ones. Several different types of structure response to compression and reverse decompression are reviewed with examples based on research of salts, co-crystals and hydrates of amino acids. Challenges and prospects of further studies are discussed.

Parametric Rietveld refinement is a powerful technique to apply directly physical or empirical equations to the refinement of in situ powder diffraction data. In order to investigate the possibilities and limitations of parametric Rietveld refinements for high pressure data four competitive crystallographic approaches were used to carry out a full structural investigation of the orthoferrite LaFeO 3 ( Pbnm at ambient conditions) under high pressure up to 47 GPa. Approach A with traditional Rietveld refinement using atomic coordinates, Approach B where the Rietveld refinement was done by using the rigid body method, Approach C where symmetry modes were used and Approach D where the newly developed method of the rotational symmetry mode description for a rigid body was used. For all approaches sequential as well as parametric refinements were carried out, confirming a second order phase transition of LaFeO 3 to a higher symmetric phase (space group Ibmm ) at around 21.1 GPa and an isostructural first order phase transition at around 38 GPa. Limitations due to non-hydrostatic conditions as well as the possibilities of a direct modeling of phase transitions with parametric Rietveld refinement are discussed in detail.

This paper reports the structural characterization of sodium azide and sodium bifluoride (hydrogenfluoride) at elevated pressures using neutron powder diffraction. Compression of sodium azide at 294 K induces a transition from the β -phase to the α -phase. This structure responds to increasing pressure by progressive tilting of the azide groups. Compression of the α -form to 3.33 GPa at 393 K results in a phase transition to the tetragonal γ -phase (space group I 4/ mcm ), which is isostructural with the azides of the heavier Group 1 elements and features square-antiprism co-ordination of the cations. On decompression at ambient temperature, the γ -phase reverts to the α -phase, but with substantial peak broadening indicative of significant strain within the recovered sample. Compression of sodium bifluoride (NaDF 2 -I to 0.66 GPa at ambient temperature resulted in the formation of NaDF 2 -II, which adopts an orthorhombic, marcasite-like (space group Pnnm ) structure. Further compression to 4.58 GPa resulted in a transition to NaDF 2 -III, which adopts the archetypal I 4/ mcm structure shared by the heavier alkali metal bifluorides and γ -NaN 3 .