High-pressure synthesis of heavily hole-doped cuprates ${\mathrm{Mg}}_{1-x}{\mathrm{Li}}_x{\mathrm{Cu}}_2{\mathrm{O}}_3$ with quasi-one-dimensional structure

We investigate electronic properties of hole-doped cuprates ${\mathrm{Mg}}_{1-x}{\mathrm{Li}}_x{\mathrm{Cu}}_2{\mathrm{O}}_3$ with the quasi-one-dimensional two-leg-ladder structure. We succeeded in extending the solubility limit of Li in ${\mathrm{Mg}}_{1-x}{\mathrm{Li}}_x{\mathrm{Cu}}_2{\mathrm{O}}_3$ from $x=0.12$ to $x=0.60$ by using the high-pressure synthesis technique. The antiferromagnetic transition temperature rapidly decreases with increasing Li content from 94 K at $x=0$ to 7.5 K at $x=0.30$, and takes an almost constant value (3–6 K) at $x\ge 0.35$. The antiferromagnetic order still exists even at $x=0.60$, where the formal valence of Cu is as large as $+2.30$. The temperature dependence of the specific heat suggests the finite contribution of the electronic specific heat at $x=0.20–0.60$, which is consistent with high valence of Cu. Nevertheless, the temperature dependence of resistivity shows a variable range hopping behavior in the whole $x$ ranges, and the insulating behavior survives under the pressure up to 2.9 GPa. This peculiar behavior is owing to the disorder originating from the intersite atom exchanges due to the similar ionic radius of cations in ${\mathrm{Mg}}_{1-x}{\mathrm{Li}}_x{\mathrm{Cu}}_2{\mathrm{O}}_3$. Nonmagnetic ions of ${\mathrm{Mg}}^{2+}$ and ${\mathrm{Li}}^{+}$ are introduced into the ${\mathrm{Cu}}_2{\mathrm{O}}_3$ planes of ${\mathrm{Mg}}_{1-x}{\mathrm{Li}}_x{\mathrm{Cu}}_2{\mathrm{O}}_3$, resulting in the localization of doped hole carriers.

Figure 1

Crystal structure of ${\mathrm{MgCu}}_2{\mathrm{O}}_3$ [(a) whole image, (b) the $ab$ plane, and (c) the $bc$ plane]. (d) X-ray diffraction patterns for polycrystals of ${\mathrm{Mg}}_{1-x}{\mathrm{Li}}_x{\mathrm{Cu}}_2{\mathrm{O}}_3$ with $x=0$, 0.20, 0.40, and 0.60. The Miller indexes on the basis of the orthorhombic $Pmmn$ are also shown. The symbols $\#$ represent peaks attributed to KCl which is derived from the oxidizing agent ${\mathrm{KClO}}_4$. Composition dependence of (e) lattice parameters ($a, b$, and $c$) normalized by the values of $a, b$, and $c$ at $x=0$ ($a_0=4.00$ Å, $b_0=9.34$ Å, and $c_0=3.19$ Å) and an unit cell volume $\left(V\right)$ on the basis of the orthorhombic $Pmmn$ space group in ${\mathrm{Mg}}_{1-x}{\mathrm{Li}}_x{\mathrm{Cu}}_2{\mathrm{O}}_3$. The dotted curves in (e) and (f) are a guide for the eye.

Figure 2

Temperature $\left(T\right)$ dependence of the resistivity $\left(\rho \right)$ for ${\mathrm{Mg}}_{1-x}{\mathrm{Li}}_x{\mathrm{Cu}}_2{\mathrm{O}}_3$ with $x=0$, 0.10, 0.20, 0.40, and 0.60. The inset shows the same data plotted against $T^{-1/2}$ and $T^{-1}$ for $x=0.40$. The plotted temperature range is 50–200 K.

Figure 3

Composition dependencies of (a) the activation energy ($E_g$) estimated from the resistivity, (b) the antiferromagnetic transition temperature ($T_{\mathrm{N}}$), and (c) the electronic specific heat coefficient $\left(\gamma \right)$ and the Debye temperature $\left({\mathrm{\Theta }}_{\mathrm{D}}\right)$ for ${\mathrm{Mg}}_{1-x}{\mathrm{Li}}_x{\mathrm{Cu}}_2{\mathrm{O}}_3$ with $(x=0–0.60)$. The $T_{\mathrm{N}}$ values are determined by the magnetic susceptibility (blue circle) and specific heat (red square). For comparison, previously reported values of $E_g$ and $T_{\mathrm{N}}$ are also plotted in (a) and (b) as green triangles [26, 27]. The dotted curves in (c) are a guide for the eye.

Figure 4

Temperature $\left(T\right)$ dependencies of the magnetic susceptibility $\left(\chi \right)$ for ${\mathrm{Mg}}_{1-x}{\mathrm{Li}}_x{\mathrm{Cu}}_2{\mathrm{O}}_3$ with $x=0$, 0.10, 0.20, 0.30, 0.40, and 0.60 at a magnetic field of ${\mu }_{0}H=1$ T. The inset shows the temperature differentials of $\chi (d\chi /dT)$. The arrows indicate the antiferromagnetic transition temperature.

Figure 5

Specific heat divided by the temperature $(C/T)$ plotted against $T^2$ for ${\mathrm{Mg}}_{1-x}{\mathrm{Li}}_x{\mathrm{Cu}}_2{\mathrm{O}}_3$ with $x=0$, 0.20, 0.35, and 0.50 under zero magnetic field. For $x=0.50$, the data under a magnetic field of ${\mu }_{0}H=9$ T are also plotted. The inset is an enlarged plot of $C$ around the antiferromagnetic transition temperature for $x=0$. The dotted curve in the inset corresponds to the contributions of phonons.

Figure 6

Temperature $\left(T\right)$ dependencies of the resistivity $\left(\rho \right)$ for ${\mathrm{Mg}}_{1-x}{\mathrm{Li}}_x{\mathrm{Cu}}_2{\mathrm{O}}_3$ with $x=0.60$ under high pressures of 0, 0.5, 1.0, 2.0, and 2.9 GPa. The inset shows the pressure $\left(P\right)$ dependence of the activation energy $E_g$.