Switchable mirrors 1±3 made of thin ®lms of the hydrides of yttrium (YH x ), lanthanum (LaH x ) or rare-earth metals exhibit spectacular changes in their optical properties as x is varied from 0 to 3. For example, a-YH x,0.23 is a shiny, hexagonally close-packed metal, b-YH 26d is a face-centred cubic metal with a blue tint in re¯ection and a small transparency window at red wavelengths, whereas hexagonally close-packed g-YH x.2.85 is a yellowish transparent semiconductor. Here we show that this concentration dependence of the optical properties, coupled with the high mobility of hydrogen in metals, offers the possibility of realtime visual observation of hydrogen migration in solids. We explore changes in the optical properties of yttrium ®lms in which hydrogen diffuses laterally owing to a large concentration gradient. The optical transmission pro®les along the length of the ®lm vary in such a way as to show that the formation of the various hydride phases is diffusion-controlled. We can also induce electromigration of hydrogen, which diffuses towards the anode when a current¯ows through the ®lm. Consequently, hydrogen in insulating YH 3-d behaves as a negative ion, in agreement with recent strong-electron-correlation theories 4,5 . This ability to manipulate the hydrogen distribution (and thus the optical properties) electrically might be useful for practical applications of these switchable mirrors.Diffusion of hydrogen in metals has attracted considerable attention 6±8

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The optical transmission of the recently discovered switchable yttrium hydride films is determined spectroscopically as a function of hydrogen content. This is done during electrochemical loading of Pd-capped Y film electrodes, thereby continuously changing the hydrogen concentration. The effect of the Pd cap layer on the film transmission is determined from measurements on a series of films with varying Pd layer thickness. The results are in good agreement with transmission measurements of in situ gas phase loaded, uncapped Y films. Both data sets can be consistently described with simple optical decay lengths such as 277.8 nm for YH 3Ϫ␦ and 15.1 nm for Pd at បϭ1.96 eV. The hydrogen concentration dependence of the optical transmission is discussed and compared with previous optical measurements on bulk samples and band-structure calculations.

As reported earlier, the metal hydride YH x reveals spectacular switchable optical properties. In this Letter we report on another remarkable property of the hydrogen deficient YH 32d trihydride phase. A logarithmic temperature dependence of the electrical resistivity is observed within a large temperature range ͑20 # T # 200 K͒ for hydrogen deficiencies 0.01 , d , 0.15. This dependence may be related to two dimensional weak localization or Kondo scattering of the conduction electrons.[ S0031-9007(97) PACS numbers: 72.15. Qm, 73.20.Fz The recent discovery of a metal-insulator transition in yttrium-, lanthanum-, and other rare earth (RE) hydride materials [1] with spectacular changes in their optical properties (YH 2 is shiny and metallic, while YH 3 is transparent and semiconducting) has revived the interest of theorists [2,3]. Until 1993 it was believed on the basis of early band structure calculations by Switendick [4] that YH 3 was a semiconductor with a gap E g ϳ 1.5 eV. More recent state-of-the-art self-consistent band structure calculations [5,6] failed, however, to reproduce a gap.Two alternative models have very recently been put forward for the YH 3 trihydride phase. Kelly et al.[2] find a complicated hexagonal structure with a lower energy than the previously assumed HoD 3 structure and a direct energy gap of 0.8 eV. Since these calculations are based on the local density approximation which typically underestimates E g by about 1 eV, the results of Kelly et al. can be considered as being consistent with our experimental findings (E g Ӎ 1.8 eV) [1,7]. Ng et al. [3] propose a model analogous to the Zhang-Rice singlet [8] for high-T c superconductors: YH 3 is a Kondo-type insulator because of electron correlation effects. The hydrogen deficient trihydride YH 32d is expected to remain insulating as long as d # d c Ӎ 0.2 because the removal of a neutral hydrogen induces the presence of a localized electron with an effective Bohr radius comparable to the lattice spacing.In order to shed some additional light on the electronic structure of YH 32d we investigated the temperature dependence of the electrical resistivity, the Hall coefficient, and the magnetoresistance as a function of the (octahedral site) vacancy concentration d. These detailed transport measurements could be carried out for the first time because our YH 32d films, in contrast with bulk YH 32d , remain structurally intact even after many hydrogen absorption and desorption cycles [9]. The transport measurements reveal a remarkable ln T dependence of the resistivity for d , 0.15 in YH 32d . We have identified two possible sources of the ln T dependence: two-dimensional (2D) localization and Kondo scattering.Yttrium films with a thickness d of 500 nm are prepared at room temperature by electron beam evaporation under high vacuum conditions (10 26 Pa). The films are covered in situ with a 5 nm thick protective palladium layer and subsequently loaded with hydrogen at room temperature. The characteristics of these films have been presented in...

Please be advised that this information was generated on 2018-05-10 and may be subject to change. j Phys. d : Appl. Phys. 28 (1995) 1195-1211. Printed in the UK T h e i n t e r p r e t a t i o n o f h a u p m e a s u r e m e n t s : a s t u d y o f t h e s y s t e m a t i c e r r o r sM IntroductionThe merits of the high-accuracy universal polarimeter (HAUP), introduced by Kobayashi and Uesu (1983), lie in the possibility of measuring very accurately and simultaneously the optical activity and birefringence of crystals. In addition, the rotation of the optical indicatrix is detected with a high sensitivity. The measurements require optically transparent, birefringent platelets of single crystals, with flat (polished) plane-parallel faces. It is very important to perform the measurements as a function of some external parameter in order to separate systematic errors from the optical properties, In the experiments described in this paper, the wavelength k of the light is the external parameter* Additional parameters can be temperature, electric fields (electro-optics) or magnetic fields (magneto-optics).The presence and nature of structural phase transitions can often be revealed by the birefringence (Ivanov 1991, Gehring 1977). The point symmetry of a specific phase can be studied by measuring the optical activity in several directions in a crystal, since optical activity is a tensorial property, which is sensitive to the symmetry (Nye 1985). The HAUP is, therefore, a powerful instrument in crystal optics.A very interesting problem in this field is encountered for incommensurately modulated crystals, with a centrosymmetric paraelectric phase (Cummins 1990). The incommensurate phase is often a (small) periodic deformation of the centrosymmetric paraelectric phase. The wavelengths of the periodic deformation is incommensurate with the lattice of the average structure. Therefore, the lattice translational symmetry is broken in at least one direction. Of course, the question arises of whether this phase can be optically active, because optical activity is forbidden in a centrosymmetric crystal (Nye 1985 In order to exclude the possibility that these differences are caused by the way of measuring or the way of interpreting the measurements, we feel . it necessary to reconsider very precisely the working principles of the HAUP method. In this paper, we re-derive rigorously the HAUP intensity formula (section 2 ). Experiments have been performed both on a centrosymmetric crystal with zero optical activity and on the non-centrosymmetric room-temperature phase of quartz, which has a large optical activity (section 3). In section 4 the obtained results are used to re consider critically the fitting procedures with which the optical properties are extracted from the measurements. Moreover, in section 5, the behaviour and origin of the systematic errors will be studied thoroughly. The quality and sensitivity of HAUP measurements is addressed in section 6 . For all aspects of the HAUP method, we discuss and take into accoun...

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Abstract. This paper reports on the measurement of the optical properties of incommensurately modulated ((CH3)4N)2CuCÎ4 by using the High Accuracy Universal Polarimeter. With this technique it is possible to measure, simultaneously, linear birefringence, linear dichroism, circular birefringence, circular dichroism and the rotation of the optical indicatrix. Two different samples are used. The orientation of the first sample allows for the measurement of the gyration tensor element #33. In the other sample #13 can be measured. The optical effects are studied as a function of temperature, both in a region of zero linear dichroism and in a region of finite linear dichroism. The measurements reveal that the crystals have a low defect concentration. The presence of the incommensurate modulation is clearly revealed by the linear birefringence. The linear dichroism, on the other hand, appears to be unaffected by the modulation. It is interesting that if one averages the effect of the incommensurate modulation a crystal structure is obtained that is believed to be orthorhombic and centrosymmetric. Two effects are observed that are forbidden by the symmetry of this average structure. One of them is the rotation of the optical indicatrix, which contradicts an orthorhombic symmetry. The other is the onset of non-zero optical activity, approximately halfway through the incommensurate phase. In centrosymmetric media this effect is not allowed to occur. The observed temperature dependence of the optical activity £ j3 differs from the behaviour measured by other authors. It is discussed whether the observed effects can be attributed to a symmetry breaking by the incommensurate modulation.

The principles of the high-accuracy universal polarimeter (HAUP) are discussed. The exact formula is derived for the intensity of the light that traverses the polarizer-sample-analyser system. For measurements with nearly crossed polarizers and oriented close to one of the extinction directions of the samples, the exact HAUP formula can be approximated. The approximate formula is used for the interpretation of the experiments. The fitting procedure that is used to derive the birefringence, optical activity, linear and circular dichroism from the data is described. It takes into account the systematic errors in the equipment due to the ellipticities of the polarizers and the so-called delta Y-error, which is caused by the eccentricities of the rotation stages that orient the polarizers. The fitting procedures are illustrated with measurements on a sample of (N(CH3)4)2ZnCl4.