FIG. 1. Anvil cell high-pressure excerted on YBa2Cu3O6+y changes the charges in the CuO2 plane. (a) Schematic of the anvil cell used for NMR; the micro-coil surrounds the single crystal of about 1 nano-L volume, and both are placed in the high pressure chamber with a ruby chip as an optical pressure gauge. (b) Sketch of the crystal structure of YBa2Cu3O6+y with highlighted bonding orbitals in one of the CuO2 planes. The charge content of these bonding orbitals can be measured with Cu and O NMR quadrupole splittings. As indicated in (c) it reflects the corresponding hole contents for Cu (n Cu ) and O (n O ), from which the NMR doping ζ follows (1 + ζ = n Cu + 2n O ).

High-pressure anvil cell nuclear magnetic resonance (NMR) studies of single crystals are challenging, but they can offer much insight into material properties. A microcoil inside the high-pressure region that encloses the crystal offers a good signal-to-noise ratio, but special care has to be taken to warrant hydrostatic conditions or to avoid rupture of the crystal or coil. By introducing precise monitoring of the height and diameter of the pressurized sample chamber, this can be ensured, and the data reveal the behavior of the sample chamber under pressure. While its total volume is given by the compression of the enclosed pressure transmitting fluid, the aspect ratio of the cylindrical chamber changes considerably. 63Cu and 17O NMR of two differently doped single crystals of YBa2Cu3O7−δ at pressures of up to about 4.4 GPa show the function of the cell, and orientation dependent spectra prove the soundness of the arrangement.

High-temperature superconducting cuprates respond to doping with a dome-like dependence of their critical temperature ( T c ). But the family-specific maximum T c can be surpassed by application of pressure, a compelling observation known for decades. We investigate the phenomenon with high-pressure anvil cell NMR and measure the charge content at planar Cu and O, and with it the doping of the ubiquitous CuO 2 plane with atomic-scale resolution. We find that pressure increases the overall hole doping, as widely assumed, but when it enhances T c above what can be achieved by doping, pressure leads to a hole redistribution favoring planar O. This is similar to the observation that the family-specific maximum T c is higher for materials where the hole content at planar O is higher at the expense of that at planar Cu. The latter reflects dependence of the maximum T c on the Cu–O bond covalence and the charge-transfer gap. The results presented here indicate that the pressure-induced enhancement of the maximum T c points to the same mechanism.

The ternary semiconductor AgInTe 2 is a thermoelectric material with chalcopyritetype structure that transforms reversibly into a rocksalt-type structure under high pressure. Nuclear magnetic resonance (NMR) is considered to provide unique insight into material properties on interatomic length scales, especially in the context of structural phase transitions. Here, 115 In and 125 Te NMR is used to study AgInTe 2 for ambient conditions and pressures up to 5 GPa. Magnetic field dependent and magic angle spinning (MAS) experiments of 125 Te prove strongly enhanced internuclear couplings, as well as a distribution of isotropic chemical shifts suggesting a certain degree of cation disorder. The indirect nuclear coupling is smaller for 115 In, as well as the chemical shift distribution in agreement with the crystal structure. The 115 In NMR is further governed by a small quadrupolar interaction (ν Q ≈ 90 kHz) and shows an orders of magnitude faster nuclear relaxation in comparison to that of 125 Te. At a pressure of about 3 GPa, the 115 In quadrupole interaction increases sharply to about 2400 kHz, indicating a phase transition to a structure with a well defined, though non-cubic local symmetry, while the 115 In shift suggests no significant changes of the electronic structure. The NMR signal is lost above about 5 GPa (at least up to about 10 GPa). However, upon releasing the pressure a signal is recovered that points to the reported metastable ambient pressure phase with a high degree of disorder.

The ternary semiconductor AgInTe2 is a thermoelectric material with a chalcopyrite-type structure, which is believed to transform into a rocksalt-type structure under high pressure. Nuclear magnetic resonance (NMR) is considered to provide unique insight into material properties on interatomic length scales, especially in the context of structural phase transitions. Here, 115In and 125Te NMR analyses are used to study AgInTe2 for ambient conditions and pressures up to 5 GPa. Magnetic field-dependent and magic angle spinning (MAS) experiments of 125Te prove strongly enhanced internuclear couplings, as well as a distribution of isotropic chemical shifts, suggesting a certain degree of cation disorder. The indirect nuclear coupling is smaller for 115In, as well as the chemical shift distribution in agreement with the crystal structure. 115In NMR is further governed by a small quadrupolar interaction (ν Q ≈ 90 kHz) and shows an orders of magnitude faster nuclear relaxation in comparison to that of 125Te. At a pressure of about 3GPa, the 115In quadrupole interaction increases sharply to about 2400 kHz, indicating a phase transition to a structure with a well-defined though noncubic local symmetry, while the 115In shift suggests no significant changes of the electronic structure. The NMR signal is lost above about 5 GPa (at least up to about 10 GPa). However, upon releasing the pressure, a signal is recovered that points to the reported metastable ambient pressure phase with a high degree of disorder.