Anisotropic thermal expansion ofAEFe₂As₂(AE= Ba, Sr, Ca) single crystals

Abstract

We report anisotropic thermal expansion of the parent, AEFe 2 As 2 (AE = Ba, Sr, and Ca), compounds. Above the structural/antiferromagnetic phase transition anisotropy of the thermal expansion coefficients is observed, with the coefficient along the a-axis being significantly smaller than the coefficient for the c-axis. The high temperature (200 K ≤ T ≤ 300 K) coefficients themselves have similar values for the compounds studied. The sharp anomalies associated with the structural/antiferromagnetic phase transitions are clearly seen in the thermal expansion measurements. For all three pure compounds the "average" a-value increases and the c-lattice parameter decreases on warming through the transition with the smallest change in the lattice parameters observed for SrFe 2 As 2 . The data are in general agreement with the literature data from X-ray and neutron diffraction experiments.Keywords: iron-arsenides; thermal expansion; anisotropy; structural phase transition;For over a year many experimental and theoretical aspects of physics of ternary AETM 2 As 2 (AE = Ba, Sr, and Ca; TM = transition metal) compounds were studied in great detail. One of the motivations for such studies was a desire to shed some light on what are the salient parameters that govern doping-or pressureinduced superconductivity [1,2,3,4,5,6,7] in these materials. The common feature of the parent compounds [8,9,10,11,12,13,14] is a distinct, moderately high temperature (above ∼ 100 K), coupled, structural/antiferromagnetic phase transition that generally is argued to be of the first order. Thermal expansion is often claimed to be uniquely sensitive to just such magnetic, structural and superconducting transitions [15]. For example, in Co-doped BaFe 2 As 2 , anisotropic thermal expansion measurements [16,17,18] have been instrumental in inferring unusually large, anisotropic, uniaxial pressure derivatives of superconducting transition temperature. For the parent AETM 2 As 2 materials though, we are aware only of data from pure BeFe 2 As 2 [16]. In this work we present and compare anisotropic thermal expansion measurements for three parent compounds, AEFe 2 As 2 (AE = Ba, Sr, and Ca). (Data for BaFe 2 As 2 are taken from [16].) For completeness, data on Sn -grown BeFe 2 As 2 [19] are included as well.Single crystals of AEFe 2 As 2 (AE = Ba, Sr, and Ca) were grown using conventional high temperature solution growth technique [20]. Details of growth conditions and physical properties of these samples are outlined in [10,13,19,21]. It should be noted, though, that BaFe 2 As 2 samples grown out of FeAs as well as Sn have been examined. The SrFe 2 As 2 and CaFe 2 As 2 samples were both grown out of Sn. The BaFe 2 As 2 /FeAs, SrFe 2 As 2 /Sn and CaFe 2 As 2 /Sn samples are considered to be pure and well ordered whereas BaFe 2 As 2 /Sn is known to

Thermal expansion of CaFe2As2: Effect of cobalt doping and postgrowth thermal treatment

“…The sharpness of the transition is consistent with it being the first order. Given that the data for x = 0 are comparable with the results for Sn flux grown CaFe 2 As 2 , 21 we conclude that small, coherent, submicron crystallites of Fe-As 18 appear to have no significant effect on the c-axis thermal expansion. The characteristic temperatures of these features are consistent with the published phase diagram.…”

“…Thermal expansion measurements were performed on pure, Sn-grown, CaFe 2 As 2 , 21 and several other iron-arsenide based superconductors. Exceptionally large, negative thermal expansion was recently observed in La-doped CaFe 2 As 2 .…”

Solidification and loss of hydrostaticity in liquid media used for pressure measurements

“…It has been suggested that in piston-cylinder cells it has some advantage over other PTM because its change in pressure with temperature is less severe (≈ 0.15 GPa between 300 K and low temperatures for P ≤ 1 GPa), and nearly independent of pressure. 20 [Note: in the course of this work we found that the drop in pressure from 300 K to low temperatures in our cell was slightly higher and varied with pressure, e.g. ΔP ≈ 0.26 GPa for P300K ≈ 0.73 GPa, 0.20 GPa for P300K ≈ 1.14 GPa, and 0.17 GPa for P300K ≈ 1.51 GPa.…”

Ultrasonic and microwave investigation of the structural and magnetic transitions inCaFe2As2andBaFe2Poirier1, Bilodeau2, Lefébvre3 et al. 2014Phys. Rev. B3010View full textAdd to dashboardCite

“…Concomitantly the attenuation data present a peak centered at 160 K superposed on a rapidly increasing background at high temperatures due to the CaF 2 delay line. Thermal expansivity can contribute only 0.5% [36] to the softening of the velocity below 170 K. This implies that the amplitude of the softening at the phase transition is practically identical for both compounds and is likely due to a similar lattice distortion. The inset of Fig.…”

“…When the high temperature wing of the peak is very abrupt, the low temperature one is much smoother and extends down to 80 K. The saturation of V /V near 140 K signals a softening of the compression modulus C 33 by magnetic and/or structural fluctuations. The steep decrease (∼2%) at 136 K cannot be attributed to the thermal expansivity along the c axis ( c/c) which only amounts to ∼0.1% [36]. So, the velocity data translate an abrupt decrease of the C 33 modulus at the phase transition.…”