High-pressure study of structural phase transitions and superconductivity in ${\mathrm{La}}_{1.48}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{0.12}\mathrm{Cu}{\mathrm{O}}_4$

We have determined the crystal structures and superconducting transition temperatures of ${\mathrm{La}}_{1.48}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{0.12}\mathrm{Cu}{\mathrm{O}}_4$ under nearly hydrostatic pressures in diamond anvil cells to $5.0\mathrm{GPa}$ and $19.0\mathrm{GPa}$, respectively. Synchrotron x-ray powder diffraction measurements were used to establish the pressure-temperature structural phase diagram. Under pressure the superconducting transition temperature increases rapidly from $T_c\approx 3\mathrm{K}$ to a maximum value of $22\mathrm{K}$ at $5\mathrm{GPa}$, a pressure slightly greater than that required to stabilize the undistorted $I\mathit{4}∕mmm$ structure in the superconducting state. Increasing the pressure further to $19\mathrm{GPa}$ leads to a decrease in $T_c$ to $\sim 12\mathrm{K}$. These results are discussed in relation to earlier high pressure measurements for similar materials.

Figure 1

(Color online) A schematic view of a $\mathrm{Cu}{\mathrm{O}}_6$ octahedron illustrating the relative orientations of the (110) and (1–10) axes of the HTT structure. The LTO1 structure is obtained by a single rotation of magnitude $\mid Q_1\mid$ or $\mid Q_2\mid$ about one of these axes (yielding one of the two orthorhombic twin structures). The LTO2 structure is obtained by simultaneous unequal rotations of magnitude $\mid Q_1\mid$ and $\mid Q_2\mid$ about both axes (where $\mid Q_1\mid \ne \mid Q_2\mid$). The LTT structure has $\mid Q_1\mid =\mid Q_2\mid \ne 0$.

Figure 2

(Top) dc volume magnetization versus temperature at $1\mathrm{G}$ applied magnetic field for the ${\mathrm{La}}_{1.48}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{0.12}\mathrm{Cu}{\mathrm{O}}_4$ polycrystalline sample studied in the present high-pressure x-ray powder diffraction experiments. Only zero-field-cooled (ZFC) data are shown. The three curves represent data from different particle size distributions obtained after passing the sample through various metal mesh sieves. (Bottom) dc volume magnetization versus temperature at $1\mathrm{G}$ applied magnetic field for the ${\mathrm{La}}_{1.48}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{0.12}\mathrm{Cu}{\mathrm{O}}_4$ single crystal studied in the present high-pressure magnetic susceptibility experiments. Zero-field-cooled (ZFC) and field-cooled (FC) data are shown. The data were not corrected for demagnetization effects. The crystal orientation with respect to the applied magnetic field was unknown.

Figure 3

(Top) X-ray powder diffraction scan and GSAS refinement of ${\mathrm{La}}_{1.48}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{0.12}\mathrm{Cu}{\mathrm{O}}_4$ at a pressure of $4.2\mathrm{GPa}$ and a temperature of $10\mathrm{K}$. The x-ray wavelength was $0.7000\left(9\right)\mathrm{\AA }$. In the upper trace the crosses are the data, and the solid line is the calculated pattern; the lowest trace is the residual. The vertical tick marks are located at the positions of (top) NaCl included in the diamond anvil cell as an internal manometer, (middle) Fe due to the diamond anvil cell gasket, and (bottom) ${\mathrm{La}}_{1.48}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{0.12}\mathrm{Cu}{\mathrm{O}}_4$. The HTT unit cell parameters at $T=10\mathrm{K}$ are $a=b=3.7429\left(2\right)\mathrm{\AA }$ and $c=13.0285\left(9\right)\mathrm{\AA }$. (Bottom) X-ray powder diffraction scan and GSAS refinement of ${\mathrm{La}}_{1.48}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{0.12}\mathrm{Cu}{\mathrm{O}}_4$ at a pressure of $2.2\mathrm{GPa}$ and a temperature of $10\mathrm{K}$. The x-ray wavelength was $0.7080\left(5\right)\mathrm{\AA }$. The symbols are the same as in the top frame. The LTO2 unit cell parameters at $T=10\mathrm{K}$ are $a=5.2934\left(3\right)\mathrm{\AA }$, $b=5.3100\left(3\right)\mathrm{\AA }$, and $c=13.0477\left(8\right)\mathrm{\AA }$.

Figure 4

(Top) Orthorhombic strain $[(b-a)∕a]$ as a function of pressure for ${\mathrm{La}}_{1.48}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{0.12}\mathrm{Cu}{\mathrm{O}}_4$. The solid lines are fits to $[(b-a)∕a]\propto {(T_0-T)}^{2\beta }$, where $T_0$ is the $\mathrm{HTT}\to \mathrm{LTO}1$ structural phase transition temperature and the exponent $2\beta =0.7$ is fixed to the value characteristic of the 3D, $n=2$, $XY$ model. (Bottom) $\mathrm{Cu}{\mathrm{O}}_6$ octahedra tilt angle as a function of pressure at a temperature of $100\mathrm{K}$. The solid line is a fit to the expression $Q_1\left(P\right)\propto {(P_0-P)}^{\alpha }$, where $Q_1\left(P\right)$ is the tilt angle, $\alpha =0.28$, and $P_0=4.2\mathrm{GPa}$.

Figure 5

The ${\left(110\right)}_{\mathrm{HTT}}$ Bragg reflection of ${\mathrm{La}}_{1.48}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{0.12}\mathrm{Cu}{\mathrm{O}}_4$ at three temperatures at a pressure of $2.2\mathrm{GPa}$. The x-ray wavelength was $0.7080\left(5\right)\mathrm{\AA }$. At $T=100\mathrm{K}$ the structure is LTO1, at $T=75\mathrm{K}$ a mixture of LTO1 and LTO2, and at $T=10\mathrm{K}$ mostly LTO2.

Figure 6

(Color online) (Top) Pressure-temperature phase diagram for ${\mathrm{La}}_{1.48}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{0.12}\mathrm{Cu}{\mathrm{O}}_4$ determined by high-pressure x-ray diffraction. The LTT phase region is shown in red, the LTO2 region in green, the LTO1 region in dark blue, and the HTT region in light blue. Note the increase ($\sim 10\mathrm{K}$ from $1\text{bar}$ to $2.2\mathrm{GPa}$) in the first-order $\mathrm{LTO}1\to \mathrm{LTO}2$ transition temperature with pressure. Above $2.2\mathrm{GPa}$ it becomes difficult to detect the presence of an $\mathrm{LTO}1\to \mathrm{LTO}2$ transition because of the increasingly small orthorhombic strain of the LTO1 structure; this region is indicated in the figure by a question mark. (Bottom) Superconducting transition temperature $\left(T_c\right)$ versus pressure $\left(P\right)$ for ${\mathrm{La}}_{1.48}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_{0.12}\mathrm{Cu}{\mathrm{O}}_4$. The color coding shows the crystal structure, as in the top frame. The numbers give the order of measurement for the two different high-pressure experiments (open circles labeled with primed numbers and filled circles labeled with unprimed numbers). The vertical error bars give the transition widths.