High-Pressure Behaviour of Prussian Blue Analogues: Interplay of Hydration, Jahn-Teller Distortions and Vacancies

Abstract

We report a high-pressure crystallographic study of four hydrated Prussian blue analogues: M[Pt(CN)6] and M[Co(CN)6]0:67 (M = Mn2+, Cu2+) in the range 0–3GPa. Mn[Co(CN)6]0:67 was<br>studied by single-crystal X-ray diffraction, whereas the other systems were only available in polycrystalline form. The Mn-containing compounds undergo pressure-induced phase transitions from Fm¯3m to R¯3 at 1.0–1.5GPa driven by cooperative tilting of the octahedral units. No phase transition was found for the orbitally disordered Cu[Co(CN)6]0:67 up to 3GPa. Mn[Co(CN)6]0:67 is significantly softer than the other samples, with a bulk modulus of 14GPa compared to 35GPa of the powdered samples. The discrepant pressure responses are discussed in terms of the presence of structural defects, Jahn-Teller distortions, and hydration. The implications for the development<br>of polar systems are reviewed based upon our high-pressure study.<br>

First-principles study of the structural, electronic, magnetic, and ferroelectric properties of a charge-ordered iron(ii)-iron(iii) formate framework

“…Metal-organic frameworks (MOFs), consisting of organic cations, a network of metal ions coordinated by organic bridging ligands, have attracted a lot of attention because of their multifunctional properties, such as ferroelectricity, magnetism, and conductivity. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] In particular, the combination of ferroelectric and magnetic orders (i.e., multiferroicity) in a single-phase material is of great technological and fundamental importance. 1-3, 13, 18-21 Furthermore, by considering their diversity in chemical compositions and structural topologies, MOFs are considered as highly promising candidates for multifunctional materials for their potential application in gas absorption and separation, ionic exchange and identification, conductive and catalytic, magnetic and even ferroelectric properties.…”

Pressure tuning the Jahn-Teller transition temperature in NaNiO$_2$

“…We now consider pressure-dependence of the bond length distortion index and effective coordination [SI Figure S8; Tables S8-10]. As in the ambient pressure measurements the bond length distortions are significantly larger in the NiO 6 compared to the NaO 6 octahedra, with the most significant variation being the increase in effective coordination (5.387(10) at 0.107(8) GPa to 5.504(13) at 4.24(5) GPa at 290 K) and decrease in bond length distortion index (from 0.0512(5) to 0.0458 (7) in the same pressure range at 290 K) of the NiO 6 octahedra in the monoclinic phase on application of pressure. NaO 6 octahedra exhibit far smaller changes in bond length distortion index and effective coordination, with the overall behaviour not seeming to exhibit a consistent change with pressure; effective coordination remains between 5.98 and 5.99 throughout the 290 K isotherm.…”

“…It indicates that the symmetry of JT-distorted octahedra increases with application of pressure in monoclinic NaNiO 2 , consistent with prior reports. [3][4][5][6][7] A previous study on LaMnO 3 attempted to extrapolate a linear fit to the pressuredependence of JT-distorted bond length, and estimated a critical pressure of ∼18 GPa. 3 Such an extrapolation could be performed for NaNiO 2 yielding a critical pressure of ∼50 GPa, converging at a Ni-O bond length of 1.85 Å at 290 K. However, this value is unlikely to be representative of the true critical pressure of the JT distortion in NaNiO 2 .…”

“…1,2 It has also been observed that application of pressure reduces the magnitude of distortion in M O 6 octahedra. [3][4][5][6][7] One well-studied material under pressure is LaMnO 3 (with JT active d 4 Mn 3+ ions). 2,3,8 At ambient pressure it adopts the perovskite structure with corner-sharing MnO 6 octahedra.…”

“…Nickelates containing JT-active d 7 Ni 3+ are far less studied than the manganates under pressure. This may be partly because many materials containing d 7 Ni 3+ octahedra do not exhibit a co-operative JT distortion, where the JT distortion is long-range ordered.…”