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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 K 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 K to 120 K, centered at T0 137K. This transition is reduced, by about 30K, by the internal chemical pressure going from LaOFeAs to NdOFeAs. On the contrary the superconducting critical temperature increases from 27K 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.

We have measured the misfit strain of the undoped parent compounds of FeAs-based superconductors RFeAsO systems by powder X-ray diffraction. The ROFeAs (R=La, Pr, Nd and Sm) powder samples have been synthesized in Bejing as described elsewhere [3][4][5]. The X-ray diffraction patterns were recorded at the x-ray Diffraction beam-line (XRD1) at the Elettra synchrotron radiation facility in Trieste. The lattice parameters of the 122" systems are taken for the literature. It is clear from the data that 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 at optimum doping the misfit strain is close to zero [39]. A large tensile misfit strain promotes the low temperature charge and spin ordering phase that competes with superconductivity, and high Tc superconductivity prevails where the misfit strain goes to zero.
In this work we have investigated in particular the stoichiometric system NdOFeAs.

Results
In the range between 300 and 200 K the XRD diffraction pattern of the NdOFeAs shows the typical tetragonal structure with P4/nmm space group. By decreasing the temperature below 200-150 K the line shows an increasing broadening that increases rapidly in the range between 150 and 137 K. The data are fitted with the Cmma space group below 137 K, where we can clearly see the splitting of show that the tetragonal to orthorhombic phase transition, centered at 137 K, is a 30 K wide transition extending from 150 to 120 K, and the 040 and 400 lines are well resolved with our experimental resolution only below 137 K.
It is known that tuning the chemical potential at an electronic topological transition (ETT) the electron gas shows a 2.5 Lifshitz electronic topological transition; the compressibility of the electron gas becomes negative, therefore the system has the tendency toward a first order electronic phase separation. The electronic instabilities at the ETT's have been widely studied in the case of one-dimensional (1D) and two dimensional (2D) single band systems with the formation of 1D CDW and 2D CDW insulating phases respectively. All layered undoped parent compounds of the FeAs-based superconductors are multiband systems. In fact all undoped parent compounds show a similar tetragonal to orthorhombic transition occurring at low temperature Ts detected by high resolution x-ray diffraction. We show in Fig. 1 the splitting of the "a" axis in stoichiometric ROFeAs "1111" systems at the structural transition from tetragonal (space group P4/nmm) to orthorhombic space group (Cmma) at low temperature, observed in the systems with R= La, Nd, Sm in agreement with previous works [3 -10]. In Fig. 1 we report the XRD results for AFe 2 As 2 "122" systems that show a similar structural transition from tetragonal ThCr2Si 2 -type, space group I4/mmm, to orthorhombic Fmmm space group [11][12][13][14][15][16][17]. In the orthorhombic phase a static stripe magnetic phase has been found. The structural transition takes place in a range of about 2 K in "122" systems, and it has been interpreted as first order transition [15][16][17] since it shows hysteretic behaviour. The structural transition in the "1111" systems shows a continuous character over a large temperature range above and below the critical temperature [5,6]. There is a strong coupling between magnetic and structural order parameters [18,19]. The spin ordering below the critical temperature Ts is driven by the low temperature orthorhombic phase and it shows a striped phase with the antiferromagnetic coupling in the direction of the long Fe-Fe bond (the orthorhombic ao axis) and ferromagnetic coupling in the direction of the short Fe-Fe bond (the orthorhombic bo axis).
The results in Fig. 1 clearly show that the critical temperature Tc of the structural phase transition decreases with decreasing the tensile strain due to lattice misfit. It is possible to see that the BaFe 2 As 2 case shows an anomalous behaviour.
The pressure-induced superconductivity in the non-superconducting compounds AeFe 2 As 2 (Ae=Ca,Sr,Ba) indicates the role of the lattice in tuning the chemical potential. Therefore the high T c phase can be reached by varying the lattice parameters (modified by the external pressure or internal pressure) and the carrier densities in the Fe 2 As 2 layers.
The pressure experiments in K 1-x Ba x Fe 2 As 2 [2] and K 1-x Sr x Fe 2 As 2 [28] show that the critical temperature is a function of both lattice parameters and charge density in the active FeAs layers and the maximum T c occurs along a line of points of charge density and pressure. There is now a large agreement that by using pressure, internal pressure (as it is shown in Fig. 1) and doping it is possible to decrease the temperature Ts of the structural and magnetic phase transition toward zero.   c) a pictorial view of tetragonal phase.