Switchable mirrors under high pressure

We discovered that metal-hydride films of yttrium, lanthanum and rare-earth metals could be reversibly and continuously switched between a shiny metal and a transparent large gap semiconductor.

An investigation of the MI-transition in these metal hydrides would not be complete without a detailed study of the insulating YH3-d with very small d . It is expected that upon the application of pressure the gap will be reduced and that the insulating YH3-d will become metallic again. This metal -> insulator -> metal transition was studied using cryogenic diamond-anvil-pressure cells.

Experimental technique: high pressure generation for switchable mirrors

In our laboratory, to generate high pressure for the investigation of switchable mirrors, we use cryogenic diamond anvil cells (DACs) [21]. Temperature is measured from a calibrated platinum resistor thermally anchored to one of the diamonds. Since at low temperatures diamond has an even higher thermal conductivity than copper there is a good thermal contact between sample and thermometer. The applied pressure is determined in situ, close to the superconducting samples, with the ruby fluorescence method. After correction for the temperature--induced shift of the ruby R1 fluorescence line [11] the calibration of Mao et al. is used.

In the middle of the culet of one diamond a film of Yttrium is deposited with a thickness of 500 nm and a diameter of 60 µ m. The Yttrium is protected from oxidation by a 20 nm thick Palladium layer[49] covering the whole culet of the diamond. Liquid hydrogen is condensed at its triple point (~ 14 K) in the gasket of the DAC[11] and serves both to provide the hydrogen for uptake by the Yttrium film and as a pressure medium. After closure of the DAC the set-up is warmed up to room temperature to enable the uptake of hydrogen.

Transport properties of the sample are determined by four point resistometry directly using a sensitive multimeter or lock-in amplifier. The electrical leads are patterned together with the sample on top of one of the diamonds.

Pressure dependence of the optical gap of YH3

Huiberts et al.[48] discovered that thin films of YH3-d can be switched between a reflecting mirror and a transparent insulator by varying their hydrogen content. Already at 1 bar H2-pressure at room temperature d ~ 0.2 is reached and YH3 is a transparent yellowish insulator. The optical gap is 2.3 eV and the electrical resistivity is higher than 1 W cm. YH3 and related switchable mirror compounds attracted recently a lot of interest not in the least because their electronic structure is still under debate. There are two lines of thought following (i) a band structure, Peierls-like model (e.g. Kelly et al.[kelly] and Ahuja et al.[ahuja]) and (ii) a strongly correlated electron model (e.g. Ng et al.[ng] and Eder et al. [eder]). Within the bandstructure school, two independent first principles calculations [kelly],[ahuja] predict a transition to a metallic YH3 phase at high pressure. According to the density functional calculation of Kelly et al.[kelly], a transition from an insulating distorted HoD3 phase ('Kelly'-phase) to a metallic undistorted HoD3 phase is expected at 15% decrease of the molar volume, corresponding to 14 GPa applied pressure. However, according to the ab initio calculation of Ahuja et al[ahuja], the undistorted HoD3-structure is insulating and stable at ambient pressure, while a phase transition to a metallic cubic phase at 1.5 GPa is predicted.

We find that the high pressure behavior is at variance with current bandstructure models. YH3 remains transparent at least up to 25 GPa, at variance with predicted gap closure[ahuja], [kelly]. At high pressure the optical gap decreases linearly extrapolating to zero for 55± 8 GPa, above which pressure a metallic state is expected. This is in disagreement with a theoretically predicted insulator-to-metal phase transition at 1.5 GPa[ahuja]. In-situ structural studies using synchrotron radiation reveal a 2% drop in the c-lattice vector at 4 GPa, while both above and below the phase transition the spectra are consistent with a hcp structure. Hence this phase transition is probably due to a rearrangement of the hydrogen lattice positions only and the predicted hcp to fcc transition is not observed.

Figure 1. Transmission photographs of 500 nm thick YH3 films at high pressure in a diamond anvil cell. There are two samples, one in the centre of the pictures and one at the top left hand corner of the gasket hole which is filled with H2. At the lower left hand corner of the gasket hole grains of ruby, used for pressure determination, are visible. Images are taken at 6 GPa (a), 13.6 GPa (b), 20 GPa (c) and 24.8 GPa (d). The sample is yellowish transparent (a) at low pressure, but the transmission shifts towards the red (b and c) to finally disappear completely from the visible spectrum (d).

 

Figure 2

The pressure dependence of the optical gap Eg is -0.020± 0.004 eV/GPa below 6 GPa and -0.043± 0.006 eV/GPa above 6 GPa. The latter fit extrapolates to a zero gap at 55± 8 GPa, where an insulator-to-metal (IM) transition is expected to take place.

 

References

The text above is a short version of the paper:

R.J. Wijngaarden, J.N. Huiberts, D. Nagengast, J.H. Rector, R. Griessen, M. Hanfland and F. Zontone, Towards a metallic YH3 phase at high pressure, Journal of Alloys and Compounds.

[kelly]P.J. Kelly, J.P. Dekker and R. Stumpf, Phys. Rev. Lett. 78, 1315 (1997)
[ahuja]R. Ahuja, B. Johansson, J.M. Wills and O. Eriksson, Appl. Phys. Lett. 71, 3498 (1997)
[ng]K.K. Ng, F.C. Zhang, V.L. Anisimov, and T.M. Rice, Phys. Rev. Lett. 78, 1311 (1997); Phys. Rev. B59, 5398 (1999)
[eder]R.Eder, H.F. Pen, and G.A. Sawatzky, Phys. Rev. B56, 10115 (1997)