Transition metal fluorides: from superconductors to multiferroics

Abstract

Transition metal fluorides represent an important family of complex solids displaying a variety of different properties and interesting phenomena. Despite their remarkable behaviour, these classes of materials have not received much attention and the rationalization of their behaviour is still lacking a systematic approach. This thesis aims to contribute to the field by examining previously unknown or understudied complex fluorides. The compounds were selected for their intriguing physical properties that range from superconductivity to multiferroism. The discovery of superconductivity in the iron pnictides sparked new interest in materials with layered ZrCuSiAs-type structure. Herein the properties of one of these systems, namely SrFeAsF, will be discussed. We have found that it behaves as a poor metal and undergoes a tetragonal (P4/nmm) to orthorhombic (Cmma) structural transition at T = 180 K, accompanied by a spin density wave in magnetic susceptibility and electrical resistivity. Below T < 150 K, the Fe moments order in antiferromagnetic spin-stripes. Electron doping with La3+ is a successful route to obtain superconducting phases, with maximum Tc = 27 K (x = 0.2). The isostructural AeMnPnF series (Ae = Sr, Ba; Pn = P, As, Sb) was also investigated to elucidate the influence of transition metal d-electrons and size effects of Ae and Pn on the physical properties. The isoelectronic replacement of Ae and Pn leads to a significant distortion in the tetragonal building blocks. All d5 Mn fluorides investigated here are insulating antiferromagnets with TN ~ 350 K. Due to the coexistence of electronic and magnetic ordering, the tetragonal tungsten bronze (TTB) materials KxM2+ xM3+ 1-xF3 (x = 0.4 – 0.6; M = transition metal) are potential multiferroics. The type of structural distortion adopted by these systems is strongly dependant on the M2+/M3+ ratio. For instance, our high-resolution diffraction study on K0.5Mn0.5Cr0.5F3 has revealed a small orthorhombic distortion, which indicates full chemical order of Mn2+ and Cr3+ on all crystallographic sites. K0.5Mn0.5Cr0.5F3 remains orthorhombic Ccc2 on cooling through the ferromagnetic transition at TN = 23 K. On heating, the structure is acentric up to T = 373 K, where a change to tetragonal P42/mbc symmetry marks the transition from ferroelectric (polar) to paraelectric (apolar) states. High-pressure diffraction experiments have shown that the Ccc2 structure is robust upon pressurization with anisotropic axial compressibility up to the maximum pressure applied p = 18 GPa. The crystal structure of related mixed-valence TTB fluoride K0.6Cr2+ 0.6Cr3+ 0.4F3 is influenced by the presence of Jahn-Teller active Cr2+. The structural analysis described here revealed the presence of a small polar monoclinic distortion (P112) providing a clear signature of full charge order (CO). On heating, the gradual loss of CO leads to two consecutive structural phase transitions to orthorhombic (Pba2, T = 423 K) and then tetragonal (P42/mbc, T = 823 K) lattices, the latter is the signature of the ferro- to paraelectric transition. Below T = 150 K, increased X-ray exposure time leads to CO-melting and the stabilization of a new, charge-disordered orthorhombic phase (Cmm2), with a phenomenology similar to the CO manganites. In highpressure diffraction experiments, a further transition to tetragonal P4bm symmetry is found at p = 6 GPa. The magnetic susceptibility points towards a complex spin arrangement, with two transitions at TN = 33 K and 6 K. The results presented herein show the richness of the structural, electronic and magnetic phase diagrams of transition metal fluorides and clearly demonstrate that systematic studies on these systems will greatly enhance our current understanding of the underlying mechanisms of important phenomena such as superconductivity and ferroelectricity.