Orthorhombic Cmma Research Articles

Pressure-induced tetragonal-orthorhombic phase transitions in CeRuPO

CeRuPO, a ferromagnetic Kondo lattice, undergoes two novel pressure-induced structural phase transitions. CeRuPO undergoes an isostructural transition to a collapsed tetragonal structure near 7.5 GPa. Since pressure suppresses the initial ferromagnetic ordering of Ce3+, our observation suggests a quantum critical point for CeRuPO near this pressure. The collapsed tetragonal phase transforms into an orthorhombic Cmma structure. This phase is isostructural with the low temperature phase of LaFeAsO, which is superconducting under high pressure. Since CeRuPO exhibits a similar relationship between the two phases of LaFeAsO, the collapsed tetragonal and orthorhombic phases of CeRuPO may adopt a novel magnetic ground state.

Superconductivity inNdFe1−xCoxAsO(0.05<x<0.20)and rare-earth magnetic ordering in NdCoAsO

The phase diagram of NdFe1-xCoxAsO for low cobalt substitution consists of a superconducting dome (0.05 < x < 0.20) with a maximum critical temperature of 16.5(2) K for x = 0.12. The x = 1 end member, NdCoAsO, is an itinerant ferromagnet (TC = 85 K) with an ordered moment of 0.30(1) BM at 15 K. Below TN = 9 K, Nd spin-ordering results in the antiferromagnetic coupling of the existing ferromagnetic planes. Rietveld analysis reveals that the electronically important two-fold tetrahedral angle increases from 111.4 to 115.9 deg. in this series. Underdoped samples with x = 0.046(2) and x = 0.065(2) show distortions to the orthorhombic Cmma structure at 72(2) and 64(2) K, respectively. The temperature dependences of the critical fields Hc2(T) near Tc are linear with almost identical slopes of 2.3(1) T K-1 for x = 0.065(2), x = 0.118(2) and x = 0.172(2). The estimated critical field Hc2(0) and correlation length for optimally doped samples are 26(1) T and 36(1) Angstrom. A comparison of the maximum reported critical temperatures of well-characterized cobalt doped 122- and 1111-type superconductors is presented.

Low temperature amorphization and superconductivity in FeSe single crystals at high pressures

In this study, we report low temperature x-ray diffraction studies combined with electrical resistance measurements on single crystals of iron-based layered superconductor FeSe to a temperature of 10 K and a pressure of 44 GPa. The low temperature high pressure x-ray diffraction studies were performed using a synchrotron source and superconductivity at high pressure was studied using designer diamond anvils. At ambient temperature, the FeSe sample shows a phase transformation from a PbO-type tetragonal phase to a NiAs-type hexagonal phase at 10 ± 2 GPa. On cooling, a structural distortion from a PbO-type tetragonal phase to an orthorhombic Cmma phase is observed below 100 K. At a low temperature of 10 K, compression of the orthorhombic Cmma phase results in a gradual transformation to an amorphous phase above 15 GPa. The transformation to the amorphous phase is completed by 40 GPa at 10 K. A loss of superconductivity is observed in the amorphous phase and a dramatic change in the temperature behavior of electrical resistance indicates formation of a semiconducting state at high pressures and low temperatures. The formation of the amorphous phase is attributed to a kinetic hindrance to the growth of a hexagonal NiAs phase under high pressures and low temperatures.

The Tetragonal to Orthorhombic Structural Phase Transition in Multiband FeAs-based Superconductors

We report the temperature-dependent x-ray powder diffraction of the FeAs-based superconductors in the range between 300 and 95 K. In the case of NdOFeAs we have detected the structural phase transition from the tetragonal phase, with P4/nmm space group, to the orthorhombic phase, with Cmma space group, over a broad temperature range from 150 to 120 K, centered at T 0∼137 K. This transition is reduced, by about ∼30 K, by the internal chemical pressure going from LaOFeAs to NdOFeAs. On the contrary, the superconducting critical temperature increases from 27 to 51 K going from LaOFeAs to NdOFeAs doped samples. The FeAs layers in all undoped “1111” and “122” systems suffer a tensile misfit strain. The tensile misfit strain is reduced in “1111” and in “122” samples and at optimum doping the misfit strain is close to zero. This result shows that the normal striped orthorhombic Cmma phase competes with the superconducting tetragonal phase. In the orthorhombic clusters the charges can move only along the stripes in the b direction and are localized by the magnetic interaction.

Refined Structure and Properties of the Layered Mott Insulator BaCoS2

BaCoS2 is orthorhombic Cmma a = 6.4413(3) Å, b = 6.4926(3) Å, c = 8.9406(3) Å, and is closely related to BaNiS2, which contains Ni-S layers separated by rock salt BaS sheets. Crystals were grown from the melt by slow cooling from 1050 to 950°C. The sulfur atoms in the Co-S layers have very anisotropic thermal displacement parameters, possibly indicating that these sulfurs are actually in a double well potential centered at a high symmetry position in the space group Cmma . BaCoS2 is found to be a paramagnetic semiconductor (Mott insulator). The magnetic properties indicate that BaCoS2 undergoes a transformation at 300 K, which is likely due to the onset of antiferromagnetic order.