Structural phase transition in two-dimensional tetramer-cuprate Na5RbCu4(AsO4)4Cl2

Abstract

Using Raman scattering and optical birefringence we have investigated a low-temperature phase transition in single crystal of the two-dimensional Na 5 RbCu 4 (AsO 4 ) 4 Cl 2 . Phonon anomalies point to a first order nature of the transition. The observed transition is most probably related to a order-disorder transition of the Rb ion positions along the z axis within the ionic framework of mixed alkali metal chloride lattices. A large part of the motivation to search and study new families of materials with small spin and low dimensionality is based on the discovery of high-temperature superconductivity in the cuprates. Following this line two-dimensional systems with pronounced magnetic fluctuations, competing interactions and structural instabilities have been discovered and investigated [1,2]. Recently a new salt-inclusion copper arsenate Na 5 RbCu 4 (AsO 4 ) 4 Cl 2 with a remarkable crystal structure was synthesized using a conventional solid-state reaction [3]. This compound exhibits a composite structure of alternating magnetic and insulating layers. The magnetic layer contains [Cu 4 O 12 ] 16-tetrameric units with Cu 4 O 4 magnetic cores, which are connected by AsO 4 bridging units. The copper valence state is 2+, so the Cu ions are magnetic with spin 1 2 . The insulating layer consists of an ionic framework of mixed alkali metal chloride lattices and rarely seen Na 6 O 8 clusters. Also the room temperature crystal structure of Na 5 RbCu 4 (AsO 4 ) 4 Cl 2 was reported as orthorhombic (space group Fmmm, Z = 2) [4]. The low-temperature crystal structure, which may have crucial implications for the distinct magnetic order, has not been determined until now. In the Ref. 5 two structural phase transitions around 74 and 110 K seen by 87 Rb nuclear magnetic resonance were reported but their nature was not clarified. PACSTo get more insight into the structural phase transitions in Na 5 RbCu 4 (AsO 4 ) 4 Cl 2 we performed Raman scattering (RS) and birefringence experiments. Experiments on the visual observations of the domain structure formed in the vicinity of the structural phase transition were carried in parallel.RS measurements were performed in quasi-backscattering geometry with the excitation line l = 514.5 nm of an Ar + laser. The laser power of 5 mW was focused to a 0.1 mm diameter spot on the sample surface. Raman spec-