Oxygen configuration and disordering in the superconductor (Hg0.7Tl0.3)2Ba2(Y0.8Ca0.2)Cu2O8+δ

PHYSICA@ ELSEVIER

Physica C 281 (1997)228-236

Oxygen configuration and disordering in the superconductor

(Hg o.7T10.3)2Ba2( Yo.8Ca 0.2)Cuz O8+8 Tomoko Ohta a, Fujio Izumi a,*, Ayako Tokiwa-Yamamoto b Keiichi Tanabe b Alan W. Hewat c a National lnstitutefor Research in Inorganic Materials, 1-1 Namiki, Tsukuba, lbaraki 305, Japan b Superconductivity Research Laboratory, International Superconductivity Technology Center, 1-10-13 Shinonome, Koto-ku, Tokyo 135, Japan c Institut Max von Laue-Paul Langevin, BP 156 F~38042 Grenoble Cedex, France

Received 23 May 1997; accepted 3 June 1997

Abstract The structure of (Hg0.TTlo.3)2Ba2(Yo.sCao.2)Cu208+ 8 with a Tc of 65 K was determined by neutron powder diffraction. Rietveld refinement based on space group I4/mmm showed a highly disordered atomic arrangement on double HgO~ + ~/z sheets. Both Hg/TI and 03 atoms on the double sheets had apparently large isotropic atomic displacement parameters, which results from incomplete modelling of their disordered configuration. The occupation factor, g, of the 03 site split into I four pieces, was reported to be 0.194 in Hg2Ba2YCu2OT.55, whereas g(O3) was very near to z in (Hgo.TT10.3)2Ba2(Yo.sCao.2)Cu2Os+~ (6 = 0). Such negligible oxygen deficiency is evidently due to partial substitution of T13+ ions for Hg 2÷ ions. Distances between apical oxygen, 02, and cations surrounding it are discussed in detail from a crystal chemistry point of view. A bond-valence sum of + 2.084 for Cu in the present oxide supports the idea that it is underdoped with hole carriers. © 1997 Elsevier Science B.V. PACS: 61.12 - q; 74.62.Bf; 74.70 - b; 74.72Gr Keywords: Crystal structure; Neutron diffraction;Rietveld analysis

1. Introduction Since the discovery of superconductivity in HgBa2CuO4+,5 [1], a considerable number of superconductors containing Hg have been found to arouse intense attention. In particular, the physical, chemi-

* Corresponding author. Fax: +81 298 527449; e-mail: [email protected]

cal, and structural properties of a homologous series of oxides with a general formula of HgBa EC a n - 1Cu nO2 n + 2 + ,5, which is conventionally referred to as H g - 1 2 ( n - 1)n, have been extensively investigated. The superconducting transition temperatures, Tc, of Hg-12(n - 1)n with n = 1 - 3 are higher than those of corresponding m e m b e r s in T1Ba 2Ca n_ l C U n O E n + 3 + , 5 : 9 6 K for Hg-1201 [1], 127 K for Hg-1212 [2], and 135 K for Hg-1223 [3]. The Tc of members in another homologous series T1EBa 2-

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T. Ohta et al./Physica C 281 (1997) 228-236

Ca~_lCunO2~+4+8 (n < 3) with double T1OI+8/2 sheets are higher than those of corresponding oxides in the T1Ba2Can_lCu~O2n+3+8 system. Thus, superconductors with double sheets of Hg plus O were of interest because of the differences in T~ between T1Ba2Can_~Cu~O2~+3+8 and T12Ba2Can_ 1Cu~O2~+ 4+ 8. However, attempts to synthesize them under ambient pressure were unsuccessful. Radaelli et al. [4,5] first succeeded in synthesizing Hg2Ba2YCu2Os+ 8 (Ca-free Hg-22t2) containing oxygen-deficient double HgO~÷8/2 sheets ( 6 = - 0 . 4 5 ) with a high-pressure, high-temperature technique. As Fig. 1 shows, Hg2Ba2YCu2Os÷ 8 is structurally related to T12 Ba2CaCu208 + 8. The two drawings in Fig. 1 are idealized as if the HgO l + 8/2 sheet had a rock-salt-like atomic arrangement with fully occupied 03 positions (tS= 0). Hg2Ba2YCu2Os+ 8 was an insulator. Replacing 30-50% of Y with Ca gave rise to superconductivity with Tc < 45 K [4], but Radaelli et al. [4] could neither obtain Hg2Ba2CaCu2Os + 8 nor optimize its hole concentration because of the solubility limit of Ca. Tokiwa-Yamamoto et al. [6,7] systematically studied synthetic conditions and superconducting

properties of (Hgl_xTlx)2Ba2(Yl_yCay)Cu2Os+ ~, i.e. (Hg,TI)-2212. They found that partial substitution of T1 for Hg stabilizes the (Hg,T1)-2212 phase under high pressure, widening the y range considerably. In oxides with x = 0.3 and 0.5, y3+ ions could be replaced by Ca 2+ ions over almost the full range of y. The doping state varied from underdoped to overdoped by increasing y in the solid solution (Hg0.TT10.3)2Ba2(Yl - y C a y ) C U 2 0 8 + 8- The highest Tc of 84 K was achieved at x = 0.3 and y = 0.4 - 0.5. Similar substitution of T1 for Hg led to a slight rise in Tc in Hg-1223 [8]. Tatsuki et al. [9-11] later prepared a series of members in the homologous series (Hgl_xTlx) 2Ba2Ca~_lCunO2n+4+8 ( n = l - 5 ) , i.e. (Hg,T1)2 2 ( n - 1 ) n . They were all superconductors except (Hg,T1)-2201. Annealing in atmospheres of Ar, H 2 (3%) + Ar, and H 2 (5%) + N 2 raised the T¢ of the members with n = 2 - 5 and x = 0.5, in particular, the 2212 and 2223 phases [11]. These experimental facts indicate overdoped states of as-prepared samples. The highest Tc was 82, 120, 115 and 95 K for the 2212, 2223, 2234 and 2245 phases, respectively. The present work aims at clarifying the structural

Cu O2 Hg 03 Ba ----~ 0 - g

~

----~

HgO6

CuO5

Fig. 1. Crystal structure of Hg2Ba2YCu2Os+ ~ [4] represented with (a) CuO 5 tetragonal pyramids and virtual HgO 6 octahedra, and (b) a ball and stick model.

T. Ohta et al. / Physica C 281 (1997) 228-236

230

properties of (Hgo.7T10.3)2Ba2(Yo.8Ca0. 2)Cu208+ (x = 0.3 and y = 0.2) by neutron powder diffraction. This chemical composition was chosen because a nearly pure phase with a fairly high Tc can be obtained there. Wu et al. [12] have recently reported X-ray Rietveld refinements of (Hg0.vTlo.3)2Ba 2( Y l _ y f a y ) C u 2 O s + ~ ( y = 0.1 - 0.6). Neutron diffraction is, nevertheless, indispensable for acquiring detailed information about the light oxygen atoms in the structure of (Hg,T1)-2212 in the presence of heavy metals such as Hg, T1, Ba and Y.

2. Experimental A precursor was prepared from BaO 2, Y203, CaO and CuO powders by heating at 900°C for 24 h in flowing 02 and then cooling in flowing O 2 at a rate of 3°C/min. A mixture of the precursor, HgO, and T1203 was reacted at 1050°C and 5 GPa for 1 h with a cubic-anvil-type apparatus according to a procedure described previously [7]. DC magnetic susceptibilities, X, of the product were measured with a SQUID magnetometer in a magnetic field of 20 Oe with a zero-field cooling mode. Fig. 2 shows the resulting X vs. temperature curve. The sharp transition and the large Meissner signal confirm the bulk nature of superconductivity for the single (Hg,T1)-2212 phase, whose onset T~ was 65 K. Neutron powder diffraction data of the sample

I

I

65 K 0 ~

-2

o

-4 -6

I

0

20

1

I

40

60

I

80

100

T~ K

Fig. 2. Temperaturedependence of the magnetic susceptibility, X, for (Hgo.TTl0.3)2Ba2(Yo.sCao.2)Cu208 + ~.

were measured at room temperature on the high-resolution powder diffractometer D2B [13] at ILL in the high-intensity mode. The white beam of neutrons from the reactor was monochromatized by a composite focusing Ge monochromator to a wavelength of 0.159432 nm. The counting rate for this sample was significantly lower than that for typical high-Tc superconductors owing to the large absorption cross section of Hg: o-a = 3.723 X 104 fm 2 [14]. The intensity data were therefore collected with a block of the as-prepared sample (about 0.4 g) supported in the neutron beam on a cadmium pin to minimize signal to noise ratios. No reflections due to impurities were observed in the diffraction pattern.

3. Rietveld refinement The structure parameters of (Hgo.7Tlo.3)2Ba 2(Y0.sCa0.2)Cu208+~s were refined with RIETAN-94 for Rietveld analysis of angle-dispersive diffraction data [15,16]. Intensity data in a 20 range of 18.22149.97 ° were analyzed. Bound coherent scattering lengths, b c, used for the refinement were 12.692 fm (Hg), 8.776 fm (T1), 5.06 fm (Ba), 7.75 fm (Y), 4.90 fm (Ca), 7.718 fm (Cu) and 5.803 fm (O) [14]. The profile shape was approximated by the pseudo-Voigt function modified by Thompson et al. [17]. Anisotropic profile broadening due to microstrain was taken into account by refining an anisotropicbroadening parameter, Yc, in addition to an isotropic-broadening one, Y, with a broadening axis of (001) [16]. Profile asymmetry was represented by employing the multi-term Simpson's rule integration [18]. The background function was a sum of Legendre polynomials (highest degree: 7) orthogonal relative to integration over the interval ( - 1, 1). Absorption was corrected for with an equation proposed by Rouse et al. [19] with a cylindrical approximation to the sample shape. Such corrections only affect the overall Debye-Waller factor for the atoms [20]. The initial structural model was based on the tetragonal space group I 4 / m m m (No. 139) with all the atoms at high-symmetry sites (Fig. 1) [4]: M1 at 4e (0,0, z; z = 0.21), Ba at 4e ( 1 / 2 , 1 / 2 , z ; z --~ 0.13), M2 at 2b ( 1 / 2 , 1 / 2 , 0 ) , Cu at 4e (0,0,z; z = 0.06),

T. Ohta et al./Physica C 281 (1997) 228-236

O1 at 8g ( 1 / 2 , 0 , z ; z = 0.05), 0 2 at 4e (0,0,z; z = 0.14), and 0 3 at 4e (1/2,1/2,z; z---0.22). M1 and M2 are mixed-metal sites having mean b c values of 0.7bc(Hg) + 0.3bc(Tl) and 0.8bc(Y) + 0.2be(Ca), respectively. Thermal vibrations of all the atoms were approximated to be isotropic. In the beginning, every site was assumed to be occupied completely. All the isotropic atomic displacement parameters, B, converged to values less than 0.015 nm 2, except an unnaturally large B value of 0.072(3) nm 2 for 0 3 on the (Hg,T1)O 1+ 8/2 sheet. Refinement of anisotropic atomic displacement parameters, Uii (i = 1 - 3), for 0 3 lowered Rwp from 7.22% to 7.01%. The resulting U/i values suggested a 'pancake-like' site, with in-plane atomic displacements much larger than off-plane ones: UI~ = U22 = 0.00135(6) nm 2 and U33 = 0.00014(5) nm 2 with an equivalent isotropic atomic displacement parameter, Beq, as large as 0.075 nm 2. This result is believed to reflect (1) displacements of 0 3 from the ideal 4e site within the ab-plane a n d / o r (2) a reduced occupancy, g, affording the apparently larger Uii parameters because of high correlations between g and Uii. In fact, 0 3 was shifted off the 4e site with 4mm symmetry along < 110 > directions with some vacancies in Hg2Ba2CaCu2Os+ ~ (6 < 0) [4,5]. Similar oxygen disorder was also reported for T12Ba2Ca ._ ~Cu,O2,+4+8 [21,22]. In view of the above results, four different models of displacements and deficiency for 0 3 [4,5] were tested in subsequent refinements. Model 1 allowed g(O3) to be varied with 0 3 still at the ideal 4e

~

231

.... ! L J !

~

Fig. 3. Difference Fourier contour map of the nuclear density on the z = ¼ plane. Step: 100 f m / n m 3 (fm: unit of be; nm3: unit of volume).

position. In addition to the refinement of g(O3), 0 3 was allowed to shift along < 110 > directions in model 2 or < 100 > directions in model 3. Thus, 0 3 was split to be situated at four equivalent positions: 16m (x,x,z) in model 2 and 16n (1/2,y,z) in model 3. In model 4, 0 3 was displaced from the 4e position without any constraints and located at the I general position 320 with a fixed occupancy of ~. Models 2 and 3 yielded better Rwp and B(O3) than model 1, with model 2 being slightly better than model 3: Rwp = 6.97% and B(O3) = 0.017(3) nm 2 in model 2; R wp = 7.03% and B ( O 3 ) = 0.028(4) nm 2 in model 3. Model 2 gave a g(O3) value of

Table 1 Fractional atomic coordinates, occupation factors, and isotropic atomic displacement parameters for (Hg07TI0.3)2Baz(YosCa0.2)CUROs+ ~

(z = 2) Atom

Site

Symmetry

x

y

z

g

B (nm 2)

MI Ba M2 Cu O1 02 03 ~

4e 4e 2b 4e 8g 4e 16m

4mm 4mm 4/mmm 4mm 2mm. 4mm ..m

0 1/2 1/2 0 1/ 2 0 0.4244(15)

0 1/2 1/ 2 0 0 0 = x

0.21249(10) 0.1258(2) 0 0.05677(12) 0.04963(11) 0.1418(2) 0.2168(4)

1 1 1 1 1 1 0.251 (7)

0.0141(5) 0.0017(8) 0.0053(10) 0.0014(5) 0.0039(5) 0.0056(7) 0.017(3)

Space group: I 4 / m m m . MI: Hg0.TTl0. 3, M2: Y0.sCa0.2. a = 0.386253(15)nm, c = 2.89629(14) nm, and V = 0.43210(3) nm 3. Rwp = 6.97%, Rp = 5.49%, R B = 6.98%, R F = 4.73%, and S = Rwp/R e = 1.42. a Split into four fractions.

232

T. Ohta et al./Physica C 281 (1997) 228-236

0.251(7) very near to ¼; that is, the deficiency at the 03 site was negligible ( 6 = 0). The refinement adopting model 4 was unstable without any significant improvement in Rwp. Despite the large B(O3) parameter, we concluded that model 2 provides the best approximation to the actual disordered configuration of 03. In model 2, B(M1) was rather large: B(M1)= 0.0141(5) nm 2. Refining the U, of M1 slightly lowered Rwp from 6.97% to 6.94%, giving UII about twice as large a s U33: UII = U22 = 0.00211(10) nm 2 and U33 = 0.0010(2) nm 2. The apparently large B(M1) value can be attributed to the positional disordering of M1 atoms within the ab-plane and/or incorporation of atoms whose bc is smaller than M1. We tested some other models for the M1 site similar to the above models for the 03 site and a new model where g(Hg) and g(T1) were varied under an equality constraint g(Hg) + g(T1) = 1. However, these refinements were always inconclusive and resulted in very large correlation coefficients between some of the refined parameters with no significant decrease in Rwp. Shimakawa et al. [23] reported the inclusion of interstitial oxygen surrounded by four T1 atoms in a distorted tetrahedral arrangement in T12 Ba2CuO6 + ~. Oxygen defects, O4, in (Hg,T1)-2212 would be accommodated, if any, at/near corresponding 4d positions ( 0 , 1 / 2 , 1 / 4 ) between a couple of HgOl+8/2

Table 2 Selected metal-oxygen distances (1), numbers of equivalent bonds (m), and a bond angle (~b) in (Hgo.TTlo.3)2Ba2(Yo.sCao.2)Cu 2O8+8 Pair

1 (nm)

m

Pair

l (nm)

m

M1-O2 M1-O3 " M1-O3 MI-O3 b M1-O3 c

0.2048(6) 0.2090(10) 0.2322(8) 0.27650(13) 0.3147(8)

1 4 4 8 2

Ba-O3 Ba-O2 Ba-O1 M2-OI Cu-O1 Cu-O2

0.2666(12) 0.27701(14) 0.2932(4) 0.2408(2) 0.19423(5) 0.2462(7)

4 4 4 8 4 1

Triplet Cu-Ol-Cu d

~b (°) 167.8(3)

MI: Hgo.7Tl0.3, M2: Yo.sCa0.2. Coordinates of atoms outside the asymmetric unit: ~ - x + 1/2,

-y+l/2,

- z+l/2;b

x-l,

y, z;C x - I ,

y-l,

z;° x + l ,

y,z.

sheets. A difference Fourier section at the z = 1 / 4 level (Fig. 3) provided no unambiguous evidence for the occupation of the 0 4 site; peaks at the 4d positions were much lower than 'ghost' peaks at the 8f positions ( 1 / 4 , 1 / 4 , 1 / 4 ) . Nevertheless, we tried to refine g(O4); B(O4) was fixed at 0.01 nm 2 arbitrarily because of the difficulty in refining both the B and g parameters of this sparingly occupied site. This refinement gave a very small g(O4) value of 0.017(13). Estimated standard deviations, o-, evaluated in Rietveld refinements are usually significantly smaller than those obtained by least-squares

300

.~ 200

+

100

0

II

20

t

IIII

'

ILl1

4'0

I

I IIIIII

'

11111111

' 60

IIIIIIIIIIIII1~11111

'

IIIIIIIIIIIIlu1111

' 8'o 20 (o)

16 0

II

IIII

'

IIIII

IIIIIIII

10

III

I|111111111

'

' 140

I

Fig. 4. Neutron Rietveld-refinement profiles for (Hgo.vTlo.3)2 Ba2(Yo.sCao. 2)Cu208 + ~. Plus symbols are observed intensity data, and the solid line is the calculated diffraction pattern. Tick marks below the profiles indicate the positions of allowed Bragg reflections for the (Hg,TI)-2212 phase. A difference curve is plotted at the bottom.

T. Ohta et al. / Physica C 281 (1997) 228-236

refinements with integrated intensities [24]. We cannot propose the existence of 0 4 interstitials owing to the inevitable underestimation of o- in Rietveld analysis and the large o" value of 0.013. A tripled tr value of 0.039 calculated according to Scott's method [25] also bears out this conclusion. After all, we adopted the structural model with M 1 at 4e, 03 at 16m, and no 0 4 at 4d. The resulting structure parameters for (Hg07T10.a)2Ba2(Y0. 8Ca0.2)Cu2Os+ ~ are tabulated in Table 1, and selected geometric parameters calculated with ORFFE [26] in Table 2. Numbers in parentheses are the ovalues of last significant digits throughout this paper. Fig. 4 shows observed, calculated, and difference diffraction patterns.

4. Discussion Notable structural differences between Hg-12(n 1)n and Hg-2212 are associated with oxygen atoms sharing the same sheet with Hg. HgO 8 sheets in Hg-12(n - 1)n contain variable amounts of oxygen defects corresponding to 6 [27-33]. The occupation factors of oxygen sites on HgO 8 sheets range from 0.063 in Hg-1201 [28], through 0.221 in Hg-1212 [32], to 0.44 in Hg-1223 [30]. By contrast, 03 is only 20-25% deficient in Ca-free and Ca-doped Hg-2212 [4]. The present Rietveld refinement further revealed that 03 vacancies are essentially absent in the present sample of (Hg,T1)-2212. Such negligible oxygen deficiency in (Hg,TI)-2212, viz., an increase in the coordination number, k, of M1 is evidently due to partial substitution of TI 3+ for Hg 2+ with a lower oxidation state. This idea is consistent with the finding that 03 positions are also fully occupied in (Hgo.7Pr0.3)2Ba2YCu208+ 8 [4,5], where Pr is believed to have an oxidation state of + 4. Dai et al. [8] reported a similar increase in the occupancy of an oxygen site on the HgO 8 sheet in Hg-1223 on partial substitution of T1 for Hg. Another pronounced structural difference between H g - 1 2 ( n - 1)n and Hg-2212 is noticed in locations of oxygen atoms on Hg-containing sheets. In Hg1 2 ( n - 1)n (n = 1 - 3) belonging to space group P 4 / m m m , oxygen atoms on the HgO 8 sheets partially occupy ideal lc positions ( 1 / 2 , 1 / 2 , 0 ) with 4 / m m m site symmetry [27-33]. If these high-sym-

233

metry positions were fully occupied (6 = 1), a rocksalt-like HgO sheet would be formed. Although slight occupation of oxygen sites at ( I / 2 , 0 , z ; z = 0) in H g - 1 2 ( n - 1)n was insisted in some papers [27,33], their existence is just as doubtful as the 0 4 site in the present oxide. As described above, 03 was displaced from the ideal 4e site to the 16m one in both Hg-2212 [4,5] and (Hg,T1)-2212. The apparently large B(O3) and B(M1) parameters are believed to result from incomplete modelling of the positional disorder on the (Hg,T1)O l + 8/2 sheet. The positional disordering of M1 and 03 is basically understood on the basis of the fact that, in the superconducting copper oxide, the overall a dimension is controlled by that of the CuO 2 sheet containing firm C u - O bonds [34]. With both M1 and 03 at the ideal 4e sites (i.e. rock-salt-like configuration), the M1-O3 bond length would be 0.2731 nm ( = a/72). This value is much larger than the sum of the effective ionic radii ( r ) [35], 0.2455 nm, for an M1 ion in six-fold coordination and an O 2- ion. Consequently, all 0 3 atoms and, to a lesser extent, M1 atoms relax from their ideal positions to form coordination polyhedra favourable to Hg 2+ and TI 3+ ions. The resulting coordination numbers of Hg and TI must be less than six in the real local structure. It would however be premature to assume the pyramidal coordination of Hg/T1 [4]; we should be conscious that 03 was displaced to the 16m site for the rough approximation to the complex local structure. The local relaxation cannot be treated satisfactorily by conventional methods of structure refinement to which space group symmetry and lattice periodicity are prerequisites. For example, the average structure of the T10 sheet obtained by least-squares refinement of structure parameters for T12BazCaCu208 does not agree with a local structure determined by pair distribution function analysis [36]. The marked displacement of 03 in (Hg,T1)-2212 is also explained in terms of the tendency for 03 atoms to be shifted toward T13+ which is ca. 14% smaller than Hg 2+ [35]. Each M1 atom has two short and opposing M 1 - O bonds: the M 1 - O 2 bond parallel with the c-axis (Fig. 5) and the M1-O3 bond connecting two HgOl+ 8 sheets. The M 1 - O 2 bond with l = 0.2048(6) nm is a little shorter than the latter M 1 - O 3 bond with / = 0.2090(10) nm. Other M1-O3 bonds

234

T. Ohta et al./Physica C 281 (1997) 228-236

Cu

HgFF1 Fig. 5. Atomic configuration around an 02 atom. Numbers attached to some bonds denote their lengths. 03 atoms are not drawn because of their poorly defined positions.

in the average structure are much longer than these two bonds, which must also be the case in the real local structure. The relative strength of the M 1 - O bonds obeys an empirical rule that the M-O(ap) bond is usually the shortest of all the M - O bonds within the MO k coordination polyhedron, where O(ap) is the so-called apical oxygen atom weakly bonded to a Cu atom on the CuO 2 sheet, and M is a metal coordinated to O(ap) on the opposite side [37,38]. In the present case, O(ap) is 02, and M is M1 (Fig. 5). The M 1 - O 2 bond with 1 = 0.2048 nm is a little longer than corresponding Hg-O(ap) bonds with l = 0.1941 - 0.1995 nm in Hg-12(n - 1)n (n = 1 3) [27-32]. Such relative elongation of the M 1 - O 2 bond can be explained on the basis of the following two trends in ionic radii: (a) For an element which can have two or more coordination numbers, its ionic radius increases with increasing coordination number because of the mutual repulsion of anions [35,39]. (b) When the covalent character is involved, the ionic radius depends to a larger extent on the coordination number [39]. The coordination number of MI in (Hg,T1)-2212 is evidently larger than that in H g - 1 2 ( n - 1)n. In addition, the Hg 2÷ ion is a typical soft acid [40], having directional H g - O bonds with a considerably covalent character. For example, increasing the coordination number of Hg 2÷ from 2 to 4 increases its r

value by as much as 39% [35]. As noted above, oxygen defects in H g - 1 2 ( n - 1)n are situated at the lc sites, which are fairly distant from 02, with much smaller occupation probabilities than in the 2212 phases. This structural feature suggests that the repulsion between 0 2 and 03 is relatively small in Hg-12(n - 1)n and that factor (a) is not so important in H g - 1 2 ( n - 1)n as in (Hg,T1)-2212. Although the real coordination number of Hg in (Hg,TI)-2212 remains unknown, the relatively long M 1 - O 2 bond in (Hg,T1)-2212 can be accounted for in this manner. Taking ~r(l) into consideration, I(M1-O2) in (Hg,T1)-2212 is comparable to l(Hg-O2) in Ca-free Hg-2212: 0.2057(7) nm [4]. The substitution of TI 3÷ for Hg 2÷ has reverse effects on I(M1-O2): (1) extension due to the increase in the coordination number of M1 and (2) shrinking ascribed to smaller r(T13+) than r(Hg ~+) [35]. Thus, effect (1) nearly offsets effect (2) to give similar M 1 / H g - O 2 bond lengths. Each Cu atom is coordinated to four O1 atoms firmly and one apical 02 atom additionally, forming an elongated tetragonal pyramid (Fig. 5). This geometric configuration displays a typical feature of the coordination for Cu in superconducting copper oxides [41]. However, the coordination behaviour of Cu in (Hg,T1)-2212 is rather different from that in Hg1 2 ( n - 1)n, as Radaelli et al. [5] pointed out. The C u - O 2 distances of 0.2462(7) nm in (Hg,T1)-2212 and 0.2469(9) nm in Hg-2212 [4] are much smaller than Cu-O(ap) ones of 0.275-0.282 nm in Hg-12(n - 1)n, whereas C u - O bonds on CuO 2 sheets have similar lengths of 0.193-0.194 nm. The shrinking of the C u - O 2 bond is accompanied by marked 'buckling' of the CuO 2 sheet with a bond angle, ~b(CuO1-Cu), of 167.8(3) ° in (Hg,T1)-2212, in contrast to ~b(Cu-O1-Cu) quite near to 180 ° in Hg-12(n - 1)n [27-33]. Radaelli et al. [29] suggested that longer Cu-O(ap) bonds afford flatter CuO z sheets and, consequently, higher Tc. The lower T~ in (Hg,T1)2212, with the optimal hole concentration [7], compared to Hg-1212 may be relevant to the difference in the flatness of the CuO 2 sheet between them. The shortening of the C u - O 2 bond and the buckling of the CuO 2 sheet in (Hg,T1)-2212 can be qualitatively interpreted in terms of the electrostatic valence rule [42]. The coordination numbers of both Hg 2+ and Ba 2÷ ions in Tl-free Hg-2212 are larger

T. Ohta et al./Physica C 281 (1997) 228-236

than those in H g - 1 2 ( n - 1 ) n , which leads to decreases in electrostatic bond strengths, s, reaching 0 2 from these cations in comparison with those in H g - 1 2 ( n - 1 ) n . The decreases in s(Hg 2+) and s(Ba 2÷) can be compensated by the approach of Cu towards 0 2 [43]. Thus, the negative charge on 02 is neutralized as far as possible by its immediate neighbours: Hg, Ba and Cu. In (Hg,T1)-2212, the partial replacement of Hg 2÷ by T13÷ has two effects of increasing the coordination number and average oxidation state of M1. The former effect decreases s(M1) while the latter one increases it. The two effects therefore cancel each other out, and the C u 0 2 bond also shrinks in (Hg,T1)-2212 with accompanying buckling of the CuO 2 sheet. The Tc vs. y curve for (Hg,TI)-2212 with x = 0.3 was parabolic with the highest Tc of 84 K at y = 0.4 - 0.5 [7]. Hence, the present sample with y = 0.2 is underdoped with hole carriers. The bond-valence sum [44], V, of Cu for the present sample is + 2.084. This value is in accord with the above experimental results but somewhat smaller than the oxidation state of Cu determined by iodometry: +2.14 [7]. It is interesting to note that the Cu-O1 bond (l = 0.19423 nm) in the present superconductor is slightly longer than that in non-superconducting Hg 2 Ba2YCu208 + (l = 0.19396 nm) [4]. Such elongation of the Cu-O1 bond obviously yielded the somewhat smaller V value than the experimental oxidation number of Cu. The increase in the oxygen content with increasing T1 content and substitution of larger Ca 2+ for smaller y3+ [35] are probably responsible for the longer Cu-O1 bond in (Hg,T1)-2212 with a larger hole concentration.

Acknowledgements

This work was partly supported by the Multi-Core Project of the Science and Technology Agency and the Industrial Science and Technology Frontier Program of NEDO. We wish to thank T. Kamiyama for helpful suggestions and S. Kumazawa for developing a pair of his programs, fousynMAC and meviusMAC, for Fourier/D synthesis and plotting contour maps. Figs. 1 and 5 were produced with ATOMS, ver. 3.1.1 by Shape Software.

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