The superconductor (Tl,Hg,Ca)₂(Ba,Sr)₂(Ca,Sr,Tl)Cu₂O_7.6

In the title 2212-type superconductor (thallium mercury calcium barium strontium copper oxide), which contains both Tl and Hg in the charge reservoir (CR), Sr is located at both alkali-earth (AE) metal sites. Ca enters the CR at the same time as Tl shares the smaller AE site, which increases the apical Cu-Cu distance significantly. The structure causes the superconducting Cu-O layers to become significantly puckered.

S1. Comment

Ever since the 2212-type superconductors were discovered, originating from Tl₂Ba₂CaCu₂O₈, which is as abbreviated 2212 (Subramanian et al., 1988), several closely related compounds have been prepared, differing in their chemical substitutions. Hg was tried at the Tl site (Bryntse, 1994) and Sr was introduced at both the Ba and the Ca site in different amounts (Maignan et al., 1995; Valldor et al., 2000). Single crystals of (Tl,Hg)₂Sr₂CaCu₂O_{8 - d} were obtained using a high-pressure–high-temperature technique (Valldor et al., 1999). The starting composition Tl_1.3Hg_0.7BaSr_1.5Ca_0.5Cu₂O_{8 - d} was also tried for a single-crystal growth and this structural study will be presented below. To insure that the analysed crystal was a representative of the whole sample, unit-cell parameters were also calculated from powder diffraction using a Guinier–Hägg camera on ground single crystals [a = 3.8353 (5) Å and c = 29.138 (6) Å]. The unit-cell parameters obtained from the single-crystal diffractometer data were a = 3.8380 (6) Å and c = 29.145 (7) Å. Ten elemental analyses (EDS) were performed on the single-crystal after diffraction data had been collected and the resulting stoichiometry [Tl_1.22 (4)Hg_0.79 (4)Ba_0.86 (3)Sr_1.48 (4)Ca_0.63 (2)Cu_2.03 (6)O_x] agreed with the starting composition (Tl_1.3Hg_0.7BaSr_1.5Ca_0.5Cu₂O_x). The only significant difference was the lower Ba content in the product balanced by a higher presence of Ca.

The results from EDS were used to generate the occupancies at the different sites in the structure, as illustrated in Fig. 1. The starting point of the fractional coordinates for all atoms was taken from the work of Subramanian et al. (1988). The reason for atoms O2 and O3 only being refined isotropically was the limited number of observed reflections with an intensity/background ratio greater than 3. The first model obtained with only (Tl,Hg) in the charge reservoir (CR) and (Ca,Sr) at the smaller alkali-earth (AE1) metal site between the Cu–O layers, resulted in unfavourable atomic displacement parameters at both sites: a large U value for (Tl,Hg) and a negative U value for (Ca,Sr). Both Tl and Hg were earlier observed at the AE1 site in a high-pressure synthesized 2212-type phase (Wu et al., 1998), however, they reported it as a new stacking order. The CR on the other hand was occupied by both Ca and Cu in another superconducting material, also formed under high-pressure conditions (Chu et al., 1997). Since our crystal was synthesized under pressure as well, Ca was introduced at the CR site at the same time as a minor amount of Tl was placed at the AE1 site. When refining the occupancies, all sites were assumed fully occupied, except for the O3 site, which ended up 78 (5)% occupied after refining it. Ba, Hg, Cu and O were all placed at their presumed sites, while Sr was placed at both AE sites with fixed values taken from EDS results. Tl and Ca are placed in the CR and at the AE1 site. The total amounts of each of the two metals were kept constant, according to EDS results, and only their relative occupations at both sites were refined, using one parameter. The metal distribution according to the refinement was (Tl_0.582 (2)Hg_0.395Ca_0.023 (2))₂(Ba_0.435Sr_0.565)₂(Sr_0.36Ca_{0.594 (5)}Tl_0.046 (5))Cu₂.

This caused both atomic displacement parameters to end up positive and reasonable. In the final model, only atom O3 had a large atomic displacement, however, in many papers, this O atom is displaced from the high-symmetry position (0, 0, z) to (x, x, z) (see, for example, Subramanian et al., 1988), which could be a reason for this relatively high value. Calculated bond lengths and angles are reasonable (Table 1). The CR metal atoms (Tl,Hg,Ca) are situated in distorted octahedra with shorter apical distances that suit the Hg²⁺ (40%), for which linear coordination is common.

From bond lengths between Cu and O, the bond-valence sum of Cu is calculated as BVS(Cu) = 2.195 (9). Assuming nominal oxidation states of all metal atoms, the calculated oxygen content would be 7.62, which agrees well with the refined value of 7.6 (2). The measured crystal was too small for resistivity of susceptibility measurements, however if the BVS of Cu is the main property connected to T_c in this system, the BVS–T_c comparison (Valldor et al., 2000) could be used to conclude that T_c should be within the range 75–90 K. From the same report, it is possible to conclude that both cell parameters and EDS analyses agree well with a T_c of about 80 K. When comparing interatomic distances in Tl₂Ba₂CaCu₂O₈ (Table 2; B, Subramanian et al., 1988) with those presented here (Table 2, A), several things are made clear. The CR in A is larger than in B, which also could be an indication of a minor Ca content at this site in A, since Ca²⁺ has a larger ionic radius compared with Tl³⁺ and Hg²⁺. The larger AE site (AE2) has shorter metal-to-oxygen bonds in A, as expected, since the site is partially occupied by Sr. An important observation is the size difference of the AE1 site, where A has longer metal-to-oxygen bonds, also due to presence of Sr. When comparing the O—Cu—O angles, it is obvious that A is more puckered than B. The puckering of the Cu–O layers and the presence of Sr at the AE1 site cause the apical Cu—Cu distance across the AE1 layer to be almost 0.2 Å longer in A than in B. A difference Fourier analysis was carried out and the maxima of 1.39 and -2.74 e Å^-3, respectively, were both found close to the AE2 site. The maxima are tolerable considering the high electron densities already present at the AE2 site. This high-pressure synthesis study agrees with previous reports concerning mixed sites (Chu et al., 1997; Wu et al., 1998), i.e. Cu as well as Ca enter the CR and Tl can be found at the AE1 site.

S2. Experimental

In order to obtain high purity starting materials, Tl₂O₃, HgO, and CuO were heated at 420 K overnight to remove moisture, while CaCO₃ was decomposed into CaO at 1270 K for 20 h. SrO and BaO, respectively, were prepared by heating their carbonates at 1320 K under dynamic vacuum (<6.0 × 10^-6 mbar; 1 mbar = 100 Pa) and the oxides formed were subsequently transported to a glove-box. All metal oxides were weighted, mixed in an agate mortar, and pelletized under inert conditions inside a glove-box. The metal-to-metal ratios were chosen to reach the nominal composition Tl_1.3Hg_0.7BaSr_1.5Ca_0.5Cu₂O_{8 - d}, aiming at placing Sr at two crystallographic sites in the 2212 structure. The pellets were placed in a BaZrO₃ crucible and placed inside a high-pressure chamber, which is further described by Morawski et al. (1997). 0.9 GPa of Ar pressure was applied before the heat treatment began. The reaction progressed in several steps; the pre-reaction heating (1190 K, 1 h), the melting (1410 K, 2 min), and the crystal growth (-24 K h^-1 to 1230 K), after which the heating was shut off to let the sample cool to room temperature. The crystals, found inside the crucible after the reaction, were all black, opaque and plate-like. Elemental analyses (EDS) of the crystals were performed inside a scanning electron microscope Jeol SEM 820 attached with a LINK AN10000 system. When refining the unit-cell parameters, X-ray powder diffraction was performed using elemental Si as internal standard in a Guinier–Hägg focusing camera (Cu Kα₁, λ = 1.5405 Å).

S3. Refinement

The reflection-to-parameter ratio was low, at 6.3 (138/22). This problem was not discovered until the crystal had been covered with gold for elemental analyses. The goodness-of-fit is slightly too high, which might be due to the problem with correctly weighting the weak reflections against the strong.

Computing details

Data collection: IPDS (Stoe, 1996); cell refinement: IPDS; data reduction: X-RED and X-SHAPE (Stoe, 1996); program(s) used to solve structure: JANA98 (Petricek & Dusek, 1997); program(s) used to refine structure: JANA98.

Figures

Fig. 1. The 2212 structure explaining the elemental distribution.

(I)

Crystal data

Ba0.87Ca0.64Cu2Hg0.79O7.56Sr1.49Tl1.21	Dx = 7.194 Mg m^−3
Mr = 930.2	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, I4/mmm	Cell parameters from 121 reflections
Hall symbol: -I 4 2	θ = 2.8–23.9°
a = 3.8380 (6) Å	µ = 55.06 mm^−1
c = 29.145 (7) Å	T = 293 K
V = 429.3 (1) Å^3	Plate, black
Z = 2	0.10 × 0.05 × 0.01 mm
F(000) = 796

Data collection

Stoe IPDS diffractometer	138 independent reflections
Radiation source: fine-focus sealed tube	121 reflections with I > 3σ(I)
Graphite monochromator	Rint = 0.046
Detector resolution: 6.0 pixels mm^-1	θmax = 23.9°, θmin = 2.8°
area detector scans	h = −4→4
Absorption correction: numerical X-RED version 1.09 (Stoe, 1997)	k = −4→4
Tmin = 0.057, Tmax = 0.210	l = −32→31
1228 measured reflections

Refinement

Refinement on F	22 parameters
R[F^2 > 2σ(F^2)] = 0.020	w = 1/[σ^2(F) + 0.000036F^2]
wR(F^2) = 0.021	(Δ/σ)max = 0.002
S = 2.41	Δρmax = 1.39 e Å^−3
138 reflections	Δρmin = −2.74 e Å^−3

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Tl	0.5	0.5	0.21267 (2)	0.0152 (2)	0.582 (2)
Hg	0.5	0.5	0.21267 (2)	0.0152 (2)	0.395
Ca	0.5	0.5	0.21267 (2)	0.0152 (2)	0.023 (2)
Ba	0.0	0.0	0.12228 (3)	0.0102 (3)	0.435
Sr1	0.0	0.0	0.12228 (3)	0.0102 (3)	0.565
Sr2	0.0	0.0	0.0	0.0060 (7)	0.36
Tl2	0.0	0.0	0.0	0.0060 (7)	0.046 (5)
Ca2	0.0	0.0	0.00	0.0060 (7)	0.594 (5)
Cu	0.5	0.5	0.05775 (5)	0.0073 (4)
O1	0.0	0.5	0.0558 (2)	0.012 (2)
O2	0.5	0.5	0.1435 (4)	0.017 (2)
O3	0.0	0.0	0.2180 (5)	0.040 (7)	0.78 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Tl	0.0169 (3)	0.0169 (3)	0.0118 (3)	0.00	0.00	0.00
Hg	0.0169 (3)	0.0169 (3)	0.0118 (3)	0.00	0.00	0.00
Ca	0.0169 (3)	0.0169 (3)	0.0118 (3)	0.00	0.00	0.00
Ba	0.0060 (4)	0.0060 (4)	0.0186 (6)	0.00	0.00	0.00
Sr1	0.0060 (4)	0.0060 (4)	0.0186 (6)	0.00	0.00	0.00
Sr2	0.003 (1)	0.003 (1)	0.011 (1)	0.00	0.00	0.00
Tl2	0.003 (1)	0.003 (1)	0.011 (1)	0.00	0.00	0.00
Ca2	0.003 (1)	0.003 (1)	0.011 (1)	0.00	0.00	0.00
Cu	0.0021 (6)	0.0021 (6)	0.0178 (9)	0.00	0.00	0.00
O1	0.005 (4)	0.013 (4)	0.020 (3)	0.00	0.00	0.00

Geometric parameters (Å, º)

Tl—O2	2.01 (1)	Ba—O3	2.79 (2)
Tl—O3	2.7183 (9)	Sr2—O1^v	2.514 (4)
Tl—O3^i	2.7183 (9)	Sr2—O1	2.514 (4)
Tl—O3^ii	2.7183 (9)	Sr2—O1^x	2.514 (4)
Tl—O3^iii	2.7183 (9)	Sr2—O1^xi	2.514 (4)
Tl—O3^iv	2.02 (2)	Sr2—O1^vi	2.514 (4)
Ba—O1^v	2.729 (5)	Sr2—O1^vii	2.514 (4)
Ba—O1	2.729 (5)	Sr2—O1^xii	2.514 (4)
Ba—O1^vi	2.729 (5)	Sr2—O1^xiii	2.514 (4)
Ba—O1^vii	2.729 (5)	Cu—O1	1.9199 (2)
Ba—O2^viii	2.784 (2)	Cu—O1^ii	1.9199 (2)
Ba—O2^ix	2.784 (2)	Cu—O1^vii	1.9199 (2)
Ba—O2^v	2.784 (2)	Cu—O1^xiv	1.9199 (2)
Ba—O2	2.784 (2)	Cu—O2	2.50 (1)
O2—Tl—O3	93.2 (4)	O1^v—Sr2—O1	99.4 (2)
O2—Tl—O3^i	93.2 (4)	O1^v—Sr2—O1^x	80.6 (2)
O2—Tl—O3^ii	93.2 (4)	O1^v—Sr2—O1^xi	180
O2—Tl—O3^iii	93.2 (4)	O1^v—Sr2—O1^vi	65.27 (11)
O2—Tl—O3^iv	180	O1^v—Sr2—O1^vii	65.27 (11)
O3—Tl—O3^i	89.82 (4)	O1^v—Sr2—O1^xii	114.73 (11)
O3—Tl—O3^ii	89.82 (4)	O1^v—Sr2—O1^xiii	114.73 (11)
O3—Tl—O3^iii	173.5 (6)	O1—Sr2—O1^v	99.4 (2)
O3—Tl—O3^iv	86.8 (4)	O1—Sr2—O1^x	180
O3^i—Tl—O3	89.82 (4)	O1—Sr2—O1^xi	80.6 (2)
O3^i—Tl—O3^ii	173.5 (6)	O1—Sr2—O1^vi	65.27 (11)
O3^i—Tl—O3^iii	89.82 (4)	O1—Sr2—O1^vii	65.27 (11)
O3^i—Tl—O3^iv	86.8 (4)	O1—Sr2—O1^xii	114.73 (11)
O3^ii—Tl—O3	89.82 (4)	O1—Sr2—O1^xiii	114.73 (11)
O3^ii—Tl—O3^i	173.5 (6)	O1^x—Sr2—O1^v	80.6 (2)
O3^ii—Tl—O3^iii	89.82 (4)	O1^x—Sr2—O1	180
O3^ii—Tl—O3^iv	86.8 (4)	O1^x—Sr2—O1^xi	99.4 (2)
O3^iii—Tl—O3	173.5 (6)	O1^x—Sr2—O1^vi	114.73 (11)
O3^iii—Tl—O3^i	89.82 (4)	O1^x—Sr2—O1^vii	114.73 (11)
O3^iii—Tl—O3^ii	89.82 (4)	O1^x—Sr2—O1^xii	65.27 (11)
O3^iii—Tl—O3^iv	86.8 (4)	O1^x—Sr2—O1^xiii	65.27 (11)
O3^iv—Tl—O3	86.8 (4)	O1^xi—Sr2—O1^v	180
O3^iv—Tl—O3^i	86.8 (4)	O1^xi—Sr2—O1	80.6 (2)
O3^iv—Tl—O3^ii	86.8 (4)	O1^xi—Sr2—O1^x	99.4 (2)
O3^iv—Tl—O3^iii	86.8 (4)	O1^xi—Sr2—O1^vi	114.73 (11)
O1^v—Ba—O1	89.48 (17)	O1^xi—Sr2—O1^vii	114.73 (11)
O1^v—Ba—O1^vi	59.70 (10)	O1^xi—Sr2—O1^xii	65.27 (11)
O1^v—Ba—O1^vii	59.70 (10)	O1^xi—Sr2—O1^xiii	65.27 (11)
O1^v—Ba—O2^viii	70.9 (2)	O1^vi—Sr2—O1^v	65.27 (11)
O1^v—Ba—O2^ix	130.0 (2)	O1^vi—Sr2—O1	65.27 (11)
O1^v—Ba—O2^v	70.9 (2)	O1^vi—Sr2—O1^x	114.73 (11)
O1^v—Ba—O2	130.0 (2)	O1^vi—Sr2—O1^xi	114.73 (11)
O1^v—Ba—O3	135.26 (12)	O1^vi—Sr2—O1^vii	99.4 (2)
O1—Ba—O1^v	89.48 (17)	O1^vi—Sr2—O1^xii	80.6 (2)
O1—Ba—O1^vi	59.70 (10)	O1^vi—Sr2—O1^xiii	180
O1—Ba—O1^vii	59.70 (10)	O1^vii—Sr2—O1^v	65.27 (11)
O1—Ba—O2^viii	130.0 (2)	O1^vii—Sr2—O1	65.27 (11)
O1—Ba—O2^ix	70.9 (2)	O1^vii—Sr2—O1^x	114.73 (11)
O1—Ba—O2^v	130.0 (2)	O1^vii—Sr2—O1^xi	114.73 (11)
O1—Ba—O2	70.9 (2)	O1^vii—Sr2—O1^vi	99.4 (2)
O1—Ba—O3	135.26 (12)	O1^vii—Sr2—O1^xii	180
O1^vi—Ba—O1^v	59.70 (10)	O1^vii—Sr2—O1^xiii	80.6 (2)
O1^vi—Ba—O1	59.70 (10)	O1^xii—Sr2—O1^v	114.73 (11)
O1^vi—Ba—O1^vii	89.48 (17)	O1^xii—Sr2—O1	114.73 (11)
O1^vi—Ba—O2^viii	70.9 (2)	O1^xii—Sr2—O1^x	65.27 (11)
O1^vi—Ba—O2^ix	70.9 (2)	O1^xii—Sr2—O1^xi	65.27 (11)
O1^vi—Ba—O2^v	130.0 (2)	O1^xii—Sr2—O1^vi	80.6 (2)
O1^vi—Ba—O2	130.0 (2)	O1^xii—Sr2—O1^vii	180
O1^vi—Ba—O3	135.26 (12)	O1^xii—Sr2—O1^xiii	99.4 (2)
O1^vii—Ba—O1^v	59.70 (10)	O1^xiii—Sr2—O1^v	114.73 (11)
O1^vii—Ba—O1	59.70 (10)	O1^xiii—Sr2—O1	114.73 (11)
O1^vii—Ba—O1^vi	89.48 (17)	O1^xiii—Sr2—O1^x	65.27 (11)
O1^vii—Ba—O2^viii	130.0 (2)	O1^xiii—Sr2—O1^xi	65.27 (11)
O1^vii—Ba—O2^ix	130.0 (2)	O1^xiii—Sr2—O1^vi	180
O1^vii—Ba—O2^v	70.9 (2)	O1^xiii—Sr2—O1^vii	80.6 (2)
O1^vii—Ba—O2	70.9 (2)	O1^xiii—Sr2—O1^xii	99.4 (2)
O1^vii—Ba—O3	135.26 (12)	O1—Cu—O1^ii	176.7 (4)
O2^viii—Ba—O2^ix	87.18 (8)	O1—Cu—O1^vii	89.952 (11)
O2^viii—Ba—O2^v	87.18 (8)	O1—Cu—O1^xiv	89.952 (11)
O2^viii—Ba—O2	154.4 (4)	O1—Cu—O2	91.7 (2)
O2^viii—Ba—O3	77.2 (3)	O1^ii—Cu—O1	176.7 (4)
O2^ix—Ba—O2^viii	87.18 (8)	O1^ii—Cu—O1^vii	89.952 (11)
O2^ix—Ba—O2^v	154.4 (4)	O1^ii—Cu—O1^xiv	89.952 (11)
O2^ix—Ba—O2	87.18 (8)	O1^ii—Cu—O2	91.7 (2)
O2^ix—Ba—O3	77.2 (3)	O1^vii—Cu—O1	89.952 (11)
O2^v—Ba—O2^viii	87.18 (8)	O1^vii—Cu—O1^ii	89.952 (11)
O2^v—Ba—O2^ix	154.4 (4)	O1^vii—Cu—O1^xiv	176.7 (4)
O2^v—Ba—O2	87.18 (8)	O1^vii—Cu—O2	91.7 (2)
O2^v—Ba—O3	77.2 (3)	O1^xiv—Cu—O1	89.952 (11)
O2—Ba—O2^viii	154.4 (4)	O1^xiv—Cu—O1^ii	89.952 (11)
O2—Ba—O2^ix	87.18 (8)	O1^xiv—Cu—O1^vii	176.7 (4)
O2—Ba—O2^v	87.18 (8)	O1^xiv—Cu—O2	91.7 (2)
O2—Ba—O3	77.2 (3)

Symmetry codes: (i) x, y+1, z; (ii) x+1, y, z; (iii) x+1, y+1, z; (iv) −x+1/2, −y+1/2, −z+1/2; (v) x, y−1, z; (vi) −y, x, z; (vii) −y+1, x, z; (viii) x−1, y−1, z; (ix) x−1, y, z; (x) −x, −y, −z; (xi) −x, −y+1, −z; (xii) y−1, −x, −z; (xiii) y, −x, −z; (xiv) −y+1, x+1, z.