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首页> 外文期刊>Acta Crystallographica, Section B. Structural science, crystal engineering and materials >Incommensurate modulation and thermal expansion of Sr_3B_(2+x)Si_(1-x)O_(8-x/2) solid solutions
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Incommensurate modulation and thermal expansion of Sr_3B_(2+x)Si_(1-x)O_(8-x/2) solid solutions

机译：Sr_3B_（2 + x）Si_（1-x）O_（8-x / 2）固溶体的调制和热膨胀不足

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摘要

Crystal structures of Sr_3B_(2+x)Si_(1-x)O_(8-x/2) solid solutions with nominal compositions x = 0.28, 0.53, 0.78 in the Sr_3B_2SiO_8–Sr_2B_2O_5 section of the SrO–B_2O_3–SiO_2 system are refined using single-crystal X-ray diffraction data. Incommensurate structure modulations are mainly associated with various orientations of corner-sharing (B,Si)-polyhedra. Preference is given to the (3 + 2)-dimensional symmetry group Pnma(0βγ)000(0βγ)000 for a single crystal compared with an alternate model of a twin formed by monoclinic components, each of them corresponding to the (3 + 1)-dimensional symmetry group P2_1(0βγ). Single-phase polycrystalline samples of solid solutions are investigated by high-temperature X-ray powder diffraction in air. Orientation preferences of the BO_3 units lead to a strong anisotropy of thermal expansion. Negative expansion is observed along the a axis over the temperature range 303–753 K. Anisotropy decreases both on heating and decreasing of the boron content.
机译：SrO–B_2O_3–SiO_2系统的Sr_3B_2SiO_8–Sr_2B_2O_5部分中标称组成为x = 0.28、0.53、0.78的Sr_3B_（2 + x）Si_（1-x）O_（8-x / 2）固溶体的晶体结构为使用单晶X射线衍射数据进行精制。不相称的结构调制主要与角共享（B，Si）-多面体的各种方向有关。与单斜晶成分形成的孪晶的替代模型相比，单晶的（3 + 2）维对称群Pnma（0βγ）000（0βγ）000更可取，每一个对应于（3 + 1维对称组P2_1 / n（0βγ）。通过在空气中进行高温X射线粉末衍射研究固溶体的单相多晶样品。 BO_3单元的取向偏好导致强烈的热膨胀各向异性。在303-753 K的温度范围内，沿a轴观察到负膨胀。各向异性在加热时和硼含量降低时均降低。

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