On the cyclability of the thermochromism in CuMoO4 and its tungsten derivatives CuMo1–xWxO4 (x < 0.12)
Résumé
A thermochromic substance changes its color when subjected to a temperature variation in a certain range. For practical reasons, the thermochromism should occur in the temperature range appropriate for desired applications. CuMoO4 exhibits thermochromism1–3 in the temperature region between ~175 and ~260 K, which depends strongly on the heating/cooling rate, and has two polymorphs, that is, the high temperature (HT) form (i.e., α form) and the low temperature (LT) form (i.e., γ form). The temperature-induced color change in CuMoO4 originates from a first order phase transition between the two structures (with ~10% shrinking of the unit cell volume at the transition).4 A recent study of tungsten-doped derivatives of CuMoO4, that is, CuMo1–xWxO4 (x < 0.12), showed that the temperature range of their thermochromism can be raised to a more easily accessible range.5, 6 These materials are thus attractive for uses around room temperature. For example, at ambient pressure, CuMo0.9W0.1O4 changes its color from red to green by warming above ~360 K and from green to red by cooling below ~275 K. Furthermore, the color change from green to red can be triggered by applying a weak pressure (e.g., finger push; Figure S1 of the Supporting Information), and the green color is regained by heating.5, 6 The α- and γ-type structures of CuMoO4 are maintained in the CuMo1–xWxO4 (x < 0.12) solid solution. It is of interest to probe how well CuMoO4 and its tungsten derivatives can withstand repeated cooling/warming (C/W) cycles without losing their thermochromism significantly. In the present work we examine this question of cyclability by performing optical absorbance (OA) at 520 nm, differential scanning calorimetry (DSC) analyses, and magnetic susceptibility measurements as well as spin dimer analysis for the α and γ structures of CuMoO4. Our study reveals that, after a number of repeated cooling/warming cycles, the α-form loses its ability to transform into the γ-form. The structural and physical properties of CuMo1–xWxO4 (x < 0.12) are not reported here because of their strong similarities to those of CuMoO4. Figure 1 shows how the OA value at 520 nm of CuMoO4 varies during the C/W cycles between 10 and 300 K with C/W rate of 1.5 K·min–1 (see Figure S2 of the Supporting Information for measurements with a 5 K·min–1 C/W rate). At high temperature, the single phase material is green so that the light of wavelength 520 nm is reflected hence falling down the OA values. When the temperature is lowered, the green to red transition occurs. Thus, the reflectivity spectrum changes with absorption ranging approximately from orange to UV, and the OA value increases.5, 6 To a first approximation, the OA value at 520 nm reflects the relative percentage of the α and γ forms in the probed sample. Once the α → γ transition is initiated by cooling, the α form can be recovered by warming. However, at the second cooling, only a part of the α form changes to the γ form. With increasing the number of C/W cycles, the amount of the α form that changes to the γ form decreases progressively so that the hysteresis loop of the OA versus T plot progressively becomes smoothed out, and finally the α form is stabilized in a wider temperature range. Experiments carried out with different C/W rates lead to the same observations except that, at a lower C/W rate, the total disappearance of the γ-form requires more C/W cycles without change in the transition temperatures.