在Cr-Te二元系中，前期研究多集中在Te含量介于50%~62% (原子分数)，主要化合物有Cr7Te8、Cr3Te4、Cr2Te3和Cr5Te8[1~3]。这些化合物通常是铁磁性的，Curie温度介于180~340 K[4~6]。它们都可看作是由具有NiAs结构的CrTe中金属Cr原子出现不同程度的有序缺位而形成。由于制备方法和热处理制度不同，这些化合物还可能有同素异形体，如Cr5Te8具有单斜和三方2种结构[7~14]。Tang等[7]用化学气相沉积加不同的热处理方法制备了单斜和三方2种结构的Cr5Te8纳米片，发现单斜Cr5Te8有巨大的反常Hall电导和反常Hall角。Huang等[8]用中子衍射实验研究了单斜Cr5Te8磁结构，发现在70~180 K具有铁磁性，磁矩方向沿c轴，在70 K以下它是倾斜磁结构，磁矩沿c轴方向是铁磁，沿ab面是反铁磁。三方Cr5Te8不仅有反常Hall效应还有拓扑Hall效应。为了解释拓扑Hall效应的起源，Wang等[9]猜测它具有复杂的标量自旋手性的非共面磁结构。然而，单晶磁化曲线等磁测量结果表明，它可能具有易c的简单铁磁结构[10]。长期以来，三方Cr5Te8的结构存在争议，基本认定其空间群为P
1 实验方法
本工作采用Te作为助熔剂生长三方结构的Cr5Te8单晶[10]。具体为：将纯度大于99.9%的Cr和纯度大于99.99%的Te按原子比15∶85混合，先装于Al2O3坩埚中，再密封于真空石英管中。将样品置于马弗炉中，并加热至1273 K，保温6 h，然后以2 K/h的降温速率冷却至803 K，再将样品从马弗炉移至离心机中离心，将单晶样品从液相中分离出来，获得的单晶样品呈片状且具有金属光泽。XRD (D8，CuKα )和NPD实验所用粉末样品由单晶研磨而成。NPD实验(约2 g样品装在直径6 mm的钒样品杆中)在澳大利亚核科学技术组织(ANSTO)的OPAL反应堆中子源上开展。室温(300 K)下，采用Echidna高精度粉末中子谱仪[15]确定晶体结构，波长0.1625 nm。采用Wombat高强度粉末中子谱仪[16]进行变温测量，波长为0.241 nm，温度范围为3.3~300 K，除最低温度3.3 K外，25~300 K每25 K测量一条谱线。用MERLIN Compact场发射扫描电子显微镜(SEM)的电子能量损失谱确定样品成分(原子分数)为：Cr 38.15%，Te 61.85%，成分为Cr4.935Te8或写成Cr1.234Te2，后文在不涉及计算结果时仍简称为Cr5Te8。
2 实验结果与讨论
Cr5Te8可形成单斜和三方结构，三方结构还有P
图1
图1
Cr5Te8室温粉末XRD谱的2个模型精修结果
Fig.1
Powder XRD spectra of Cr5Te8 at room temperature refined with two models (Inset is the partial enlargement of model 2, showing some inconsistencies between the model and the experiment; Iobs, Ical—observed intensity and calculated intensity, respectively)
图2
图2
Cr5Te8室温NPD谱的2个模型精修结果
Fig.2
Neutron powder diffraction (NPD) spectra of Cr5Te8 at room temperature refined with two models
表1 2个模型X射线衍射和中子衍射的精修结果
Table 1
| Diffraction pattern | Space group | a / nm | c / nm | Rp / % | Rwp / % | χ2 |
|---|---|---|---|---|---|---|
| XRD | P | 0.3900 | 0.5989 | 2.42 | 3.47 | 5.21 |
| P | 0.7800 | 1.1979 | 2.70 | 3.93 | 6.70 | |
| NPD | P | 0.3900 | 0.5986 | 1.13 | 1.42 | 1.07 |
| P | 0.7801 | 1.1972 | 1.14 | 1.43 | 1.09 |
Note: a, c—lattice parameters; Rp—prefile R factor; Rwp—weighted profile R factor; χ2—discrepancy factor
在Wombat高强度谱仪上完成了变温中子衍射研究。图3选放了300、200、100、3.3 K 4条谱线。可以看出，大多数谱线峰位没有明显移位，只有(103)峰随温度的降低右移。随着温度降低到Curie温度240 K以下，磁衍射的贡献导致在300 K没有出现(100)峰，此外大多数衍射峰出现不同程度的加强，其中(101)和(201)峰增强更明显，由于没有更多的新的衍射峰出现，说明磁增殖矢量k为(0, 0, 0)。用Bilbao Crystallographic Server的最大磁空间群MAXMAGN[17]和Fullprof Suite[18]内置的BasIreps进行不可约分析。假设磁性原子Cr1 1a(0, 0, 0)和Cr2 1b(0, 0, 0.5)磁矩m大小相等，考虑m//[001]轴，和在ab面内m//[100]，结合Cr1和Cr2磁矩平行(FM)和反平行(AFM)排列共4种情况。图4最下面谱线是3.3 K实验NPD谱减去拟合的背底和核衍射得到的，表示实验磁衍射，上面4条是4种模型计算的磁衍射。发现磁矩m/[001] FM排列时谱线能得到很好的拟合，因此Cr5Te8是一个简单共线铁磁结构，磁矩沿c轴，磁空间群是P
图3
图4
图4
Cr5Te8实验磁衍射与4种磁结构模型
Fig.4
Cr5Te8 experimental magnetic contribution and four magnetic-structure models (FM and AFM stand for ferromagnetic and antiferromagnetic order, respectively; IMexp is the experimental intensity of magnetic diffraction)
图5
图5
Cr5Te8 3.3 K NPD谱及其精修结果，及Cr5Te8磁结构示意图
Fig.5
Cr5Te8 NPD spectra at 3.3 K and the refinement (a), and schematic of the magnetic structure of Cr5Te8 (b)
从变温NPD谱拟合得到的晶胞参数和磁矩随温度的变化如图6所示。可以看出，Cr5Te8点阵参数a有小的负膨胀，点阵参数c是正常的热膨胀，以最低温度3.3 K为参考温度，在3.3~300 K之间，a方向平均膨胀系数为-10.7 × 10-6 K-1，c方向平均膨胀系数为52.4 × 10-6 K-1。由图6a和b中的点阵参数计算的Cr5Te8的(101)、(302)、(201)晶面间距随温度的变化见图7。这些方向的单晶或取向样品有近零的热膨胀，其膨胀系数分别为4.6 × 10-6、-2.9 × 10-6和-6.1 × 10-6 K-1，在3.3~300 K可望有宽温区的零膨胀应用。均质材料中的反常热膨胀有结构和电子2种起源，结构起源是指源于晶格原子势和振动(声子)，电子起源是指和电子关联相关，如磁性、铁电、超导、电荷转移材料及其相变产生的反常热膨胀[19~24]。Cr5Te8虽然是一种金属磁性材料[9]，但其热膨胀规律在Curie温度前后的顺磁态和铁磁态基本相同，没有可观测到的变化，因此其反常热膨胀应该不是电子起源，而是与其有大量Cr空位的点阵相关，属结构起源。其他有Cr空位的Cr-Te化合物中也有反常热膨胀的报道[25]。
图6
图6
Cr5Te8点阵参数a、c、a / c比和晶胞体积及Cr磁矩随温度的变化规律
Fig.6
Temperature dependences of Cr5Te8 lattice parameter (TC—Curie temperature)
(a) a (b) c(c) cell volume and a / c (d) Cr moment
图7
图7
Cr5Te8 (101)、(201)、(302)晶面间距随温度的变化
Fig.7
Temperature dependences of interplanar spacing (d) of Cr5Te8 (101), (201), and (302) planes
3 结论
用自助溶剂法生长的Cr5Te8单晶具有三方P
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