Synthesis, Structure, and Compositional Tuning of the Layered Oxide Tellurides Sr2MnO2Cu2–xTe2 and Sr2CoO2Cu2Te2

The synthesis and structure of two new transition metal oxide tellurides, Sr2MnO2Cu1.82(2)Te2 and Sr2CoO2Cu2Te2, are reported. Sr2CoO2Cu2Te2 with the purely divalent Co2+ ion in the oxide layers has magnetic ordering based on antiferromagnetic interactions between nearest neighbors and appears to be inert to attempted topotactic oxidation by partial removal of the Cu ions. In contrast, the Mn analogue with the more oxidizable transition metal ion has a 9(1)% Cu deficiency in the telluride layer when synthesized at high temperatures, corresponding to a Mn oxidation state of +2.18(2), and neutron powder diffraction revealed the presence of a sole highly asymmetric Warren-type magnetic peak, characteristic of magnetic ordering that is highly two-dimensional and not fully developed over a long range. Topotactic oxidation by the chemical deintercalation of further copper using a solution of I2 in acetonitrile offers control over the Mn oxidation state and, hence, the magnetic ordering: oxidation yielded Sr2MnO2Cu1.58(2)Te2 (Mn oxidation state of +2.42(2)) in which ferromagnetic interactions between Mn ions result from Mn2+/3+ mixed valence, resulting in a long-range-ordered A-type antiferromagnet with ferromagnetic MnO2 layers coupled antiferromagnetically.

A note on the low temperature magnetic susceptibility of the Mn-containing samples.
In Figure 4(a) of the main text of the paper we show the low temperature behaviour of the magnetic susceptibilities of the Mn-containing compounds. Given the mixed-valent nature of both compounds it seems likely that the divergence of the ZFC and FC curves may be the result of disorder and frustration of magnetic interactions leading to a spin glass component to the magnetism. However we cannot rule out that this behaviour stems from an impurity. A reviewer highlighted that MnO nanoparticles of ~5nm diameter appear to behave as ferromagnetic systems (Lee et al., J. Am. Chem. Soc. 124, 12094 2002), although with a small moment of 0.08 B per MnO formula unit in a 50000 Oe field), and noted that we indeed we see a MnO impurity. As the plots in Figure 7 show this manifests as an antiferromagnet and Wang et al. (Phys. Rev. B 83, 214418, 2011) showed that small MnO nanoparticles (~10 nm) retain the bulk antiferromagnetic properties expected for bulk MnO. Given the lack of resemblance of our low temperature susceptibility curves to the apparent ferromagnetic behaviour of MnO nanoparticles and the fact that such nanoparticles have been shown to be antiferromagnetic like our MnO impurity, it seems unlikely that an MnO phase is responsible for our observations. S7 Figure S8. The M vs H curve of parent Sr2MnO2Cu1.82(2)Te2 and I2-treated Sr2MnO2Cu1.58(2)Te2, measured at 300 K. The inset emphasises the magnetisation at low field. Figure S9. The magnetic susceptibility and inverse magnetic susceptibility (with linear fit) of a) parent Sr2MnO2Cu1.82(2)Te2 and b) I2-treated Sr2MnO2Cu1.58(2)Te2, obtained using a 40 -30 kOe subtraction. The Curie and Weiss constants were calculated using these data.

Warren-like peak-shape function
The following function, obtained from refs. 17 and 18 (main paper) was used to model the peakshape of the magnetic peak identified as being Warren-like. The function gave a reasonable qualitative fit to the data (main paper, Figure 6).
(1) S8 where the Gaussian and Lorentzian parts describe the long-range in-plane correlation and shortrange correlation along lattice parameter c respectively, ⊥ represents the in-plane peak width, ξ is the correlation-length and τ is the position of the centre of the peak. Table S3. The values obtained from fitting of the magnetic peak in Sr2MnO2Cu1.82(2)Te2 to the Warrenlike peak-shape function, with fitting statistics. Model Warren-like 0.92173 Adj. R-Square 0.92086 Figure S10. Comparison of the raw PND plots of parent Sr2MnO2Cu1.82(2)Te2, when it was measured at 1.7 K, 55 K (temperature not equalised) and 100 K. The 2/9 bank (58.3°) bank of WISH is shown. Figure S11. a) Comparison of the raw PND plots of I2-treated Sr2MnO2Cu1.58(2)Te2, when it was measured at 1.7 K, 50 K and 140 K. The 4/7 (121.7°) bank of WISH is shown and b) the refined ordered moment at 1.7 K, 50 K and 140 K.
S9 Figure S12. The two magnetic symmetry modes used to construct the magnetic ordering model of Sr2CoO2Cu2Te2. The axis labels refer to the magnetic unit cell and for clarity only the cobalt (small blue circles) and oxygen (large red atoms) are shown.
Figure S13. Contrast of the two possible magnetic models for Sr2CoO2Cu2Te2 used here: collinear and non-collinear. The collinear model was selected for use in the refinement of the magnetic intensity of Sr2CoO2Cu2Te2. The axis labels refer to the magnetic unit cell and for clarity only the cobalt (small blue circles) and oxygen (large red atoms) are shown. S10 Figure S12. Raw powder neutron diffraction patterns of Sr2CoO2Cu2Te2, measured as a function of temperature. The 2/9 (53.1°) databank of WISH is shown. Successive diffractograms are offset by 1.2 counts from the origin for clarity.  Figure S15. Rietveld plot of Sr2CoO2Cu2Te2 measured using the MAC detector at the I11 beamline at 100 K. Rietveld analysis was performed using the orthorhombic Immm model. Figure S16. Definition of the Ch-Cu-Ch (where Ch = S, Se, Te) angles  and  used in the main text.