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. 2016 Jul 27;9(8):631.
doi: 10.3390/ma9080631.

Negative Thermal Expansion in Ba0.5Sr0.5Zn₂SiGeO₇

Christian Thieme  1 , Christian Rüssel  2
Affiliations

Negative Thermal Expansion in Ba0.5Sr0.5Zn₂SiGeO₇

Christian Thieme et al. Materials (Basel). .

Abstract

Solid solutions with the composition Ba0.5Sr0.5Zn₂Si2-xGexO₇ and BaZn₂Si2-xGexO₇ were prepared with different values of x using a conventional mixed oxide route. Both compounds exhibit very different thermal expansion, which is due to the different crystal structures. Ba0.5Sr0.5Zn₂Si2-xGexO₇ solid solutions exhibit the structure of high-temperature BaZn₂Si₂O₇ and show negative thermal expansion, which was proven via high-temperature X-ray diffraction. Up to around x = 1, the crystal structure remains the same. Above this value, the low-temperature phase becomes stable. The Sr-free solid solutions have the crystal structure of low-temperature BaZn₂Si₂O₇ and show also a limited solubility of Ge. These Sr-free compositions show transitions of low- to high-temperature phases, which are shifted to higher temperatures with increasing Ge-concentration.

Keywords: X-ray diffraction; negative thermal expansion; phase transition.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Results from X-ray diffraction (XRD) recorded at room temperature. (a) XRD patterns within the solid solution series Ba0.5Sr0.5Zn2Si2-xGexO7 with different values of x are shown. In the lower part, the theoretical peak positions of crystals with the structure of high-temperature (HT)- and low-temperature (LT)-BaZn2Si2O7 taken from references [3,11] are displayed; (b) the lattice parameters of Ba0.5Sr0.5Zn2Si2-xGexO7 are shown as a function of x. The values for x = 0 were taken from reference [9].
Figure 2
Figure 2
Relative change of the lattice parameters of Ba0.5Sr0.5Zn2SiGeO7 determined with HT-XRD. The values a0, b0, c0, and V0 were determined at room temperature. The line, which fits the change of the volume of the unit cell was calculated on the basis of the regression parameters given in Table 1.
Figure 3
Figure 3
Phase transition temperatures TPT of BaZn2Si2-xGexO7 solid solutions with different values of x. (a) the DSC curves are illustrated together with the respective onset temperatures of the phase transition; (b) the phase transition temperatures are plotted in dependence of x. The linear regression (dashed line and formula) is also inserted.
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