Previous High Temperature Studies

Despite the importance of studying the structure of the Bi-systems at elevated temperatures, there have been remarkably few published investigations concerned with this problem. This is perhaps because of the greater practical difficulties of high-temperature in situ observations when compared to the more usual low temperature studies which are so routine in the high-T$_c$ field. There have been two previous single crystal x-ray studies: both however suffered shortcomings including poor sample quality and low resolution.

The most direct attempt to establish the role played by oxygen content in the modulation of the Bi-systems was carried out by Shen [107]. In situ x-ray measurements were made of a highly oriented powder sample of Bi-2201 heated up to 800$^o$C. A flowing oxygen atmosphere was first used to maintain the oxygen content of the sample. The satellite intensities were observed to decrease by 30% over the range from room temperature to 800$^o$C whilst the fundamental reflections were enhanced. The atmosphere was then switched from oxygen to nitrogen at 800$^o$C, initiating a loss of oxygen, and over a period of an hour the satellites decreased by over 75% in intensity, they broadened, and their positions moved towards that of the fundamental reflections. Upon returning to an oxygen atmosphere the satellites were restored to their original positions. Longer anneal times in nitrogen led to the decomposition of the sample. The study demonstrates that oxygen content does affect both the intensity and positions of the satellite reflections. Shen [107] further explored this with ex situ annealing of samples to remove oxygen in a more controlled manner. Annealing for 10 hours produced a change in oxygen measured to be $\Delta\delta=0.043$ oxygen per formula unit and the accompanying change in the modulation wavevector was measured to be ${\bf q}=0.183{\bf b}^* + 0.315{\bf c}^*$ to ${\bf q}=0.184{\bf b}^* + 0.26{\bf c}^*$. So, surprisingly they found no change in the ${\bf b}^*$ component of ${\bf q}$ as oxygen is removed, the periodicity within individual BiO-SrO-CuO$_2$-SrO-BiO slabs remaining unchanged, with only the stacking of the modulation changing. In the Le Page model, where it is the periodic insertion of extra oxygens alone which determine q then $\beta{\bf b}^*$ and not $\gamma{\bf c}^*$, would be expected to have shown the strongest variation in this experiment. Notably, this led Shen [107] to conclude that the results did not support the extra oxygen model.

One other, less successful, attempt by Bdikin [92] looked at a ${\rm Bi_2Sr_2CaCu_2O_{8+\delta}}$ crystal in situ up to 700$^o$C and observed a gradual intensity decrease for the satellites, but the crystal was not of sufficient quality to allow any precise measurements of the satellite positions. The only notable observation was the apparent total disappearance of the diffuse streaks around 450$^o$C, a disappearance which was found to be reversible upon cooling. Unfortunately no details of the vacuum conditions or annealing times are given and so no conclusions can be made as to the possible role oxygen diffusion played in the change. Bdikin [92] wrongly attributed the diffuse streaks to a monoclinic distortion of the usually orthorhombic cell, as was discussed in Chapter 3, and hence also wrongly describes this change as a phase transition from a primitive to a face-centred monoclinic cell.

By far the greatest proportion of the published results have utilised electron microscopy techniques. Although samples may be heated in situ in the microscope in TEM they suffer the difficulty of possible electron beam damage at high temperatures and so the beam must be switched off during heating. This limits the observations to such effects as remain once the sample has been cooled. In cases where high-temperature observations have been attempted, additional heating of the sample by the electron beam and the possibility of beam induced effects must always be considered. Absolute intensity values of reflections are also difficult to obtain, and the experiments are generally limited to the [1 1 0] diffraction plane.

The earliest TEM study, by Chen [106,101], was of two ${\rm Bi_{2-x}Pb_xSr_2CaCu_2O_{8+\delta}}$ samples with $x=0$ and 0.2. Electron diffraction patterns were taken right up to temperatures close to amorphisation at 800$^o$C. Two changes in the patterns were distinguished. The first showed the appearance of weak superstructure spots at 2a, 2b positions at temperatures above 290$^o$C. The main and satellite reflections showed little appreciable change up to 490$^o$C, after which the disappearance of both satellite and the newly formed superstructure reflections was closely followed by amorphisation. The only difference for the $x=0.2$ Pb doped sample was the earlier onset of amorphisation at 470$^o$C.

Results of a similar nature have also been obtained by Zandbergen [108] on samples of ${\rm Bi_{2-x}Pb_xSr_2YCu_2O_{8+\delta}}$. The 2$a$, 2$b$ superstructure appeared in the range 450-500$^o$C for the composition $x=0.03$ and 375-420$^o$C for $x=0.6$ and 1.0. Amorphisation ranges were 700-740$^o$C and 550-600$^o$C respectively. They noted that the superstructure did not disappear upon cooling, which they took to indicate that an irreversible chemical change must be the cause. This was confirmed to be oxygen related by ex situ heating in oxygen at 450$^o$C which produced no signs of the superstructure. The superstructure disappeared 40$^o$C before amorphisation whilst the incommensurate satellites remained present right up to amorphisation. The apparent lack of influence that the formation of the superstructure had upon the satellites led them to postulate that the superstructure must, most probably, be localised within the Sr-O-Cu perovskite block.

Perhaps the most illuminating of the TEM results were those obtained by Gao [109], and later Yang [110]. Annealing ${\rm Bi_2Sr_2CaCu_2O_{8+\delta}}$ at 350-400$^o$C in vacuum, Yang [110] observed after returning the sample to room temperature, twin boundaries in HREM images along the [0 1 0] direction and the associated splitting of fundamental reflections in the [1 0 0] direction. The formation of the twins required at least 30 minutes anneal time. The observations imply a rotation of (0 k 0) planes by 1.3 degrees across the twin boundaries. Again, as in the previous studies, the satellites were unaffected, and were observed to persist up to about 500$^o$C. On this basis they suggested the diffusion of oxygen out of the crystal does not therefore take place from the oxygen-rich Bi$_2$O$_2$ layers. But rather the twinning is associated with loss of oxygen and an ordering of vacancies in the CuO$_2$ plane. Gao [109] observed identical twinning in as-grown samples of Pb doped Bi-2212, and proposed a monoclinic distortion associated with either cation or oxygen ordering. The high value of T$_c$=90K in these samples, however, would seem to make it unlikely that oxygen vacancies within the CuO$_2$ layers could be present as proposed by Yang [110]. In both studies, annealing in air removed any sign of twinning.

All of these studies do, therefore, demonstrate that the removal of oxygen by high-temperature annealing proliferates significant structural modifications. But, that, whatever the nature of these modifications the modulation appears to remain impervious to them. In all of the studies, the modulation persisted unchanged until very close, to within 50$^o$C, of the amorphisation temperature, suggesting that the modulation does not begin to disappear until the fundamental structure itself becomes unstable. Even where the satellites were observed to change, as in the most direct study by Shen, the alteration was still only observed just prior to the decomposition of the sample. The failure of these studies to detect any variation in the $\beta{\bf b}^*$ component of the modulation with oxygen content is of further concern when variation in the incommensurability is widely discernible between samples grown with different cation compositions. The evidence then suggests that the oxygen-structure relationship in the Bi-systems is a complicated one, and a deeper understanding of it is required before any judgements may be drawn conclusively about the role of oxygen in the modulation.