Results

Calculations were carried out for the standard defect C_i, and C_i-P, the T-centre (C_s-C_iH), H at a bond centred site (H$_{\rm BC}$), H at anti-bonded lattice site, H$_{\rm AB}$, VO, VP, V₂, Au, Ag, Pt, Pd, AuH_n, AgH_n, PdH_n and PtH_n and the results are given in Table 1 where they are compared with experimental values.

In general the calculated electrical levels of light impurities are within about 0.2 eV of the experimental results where available. Whereas the donor and acceptor levels of C_s-C_iH, C_iP, VP, VO and even V₂ are within 0.2 eV of the experimental values, it is surprising that the 0/+ level of H$_{\rm BC}$ is found so close to the observed level given that the theory is expected to be worse when the level moves far away from that of C_i. For Ag, Au, Pd and Pt, all in low spin configurations, we find the (-/0) and (0/+) levels to be deep in the gap and within 0.2 eV of the observed values. We now investigate the effect of adding H atoms to these defects.

 

Table 1: Electrical levels, eV, of deep centres. (0/+) is referred to E_v and (-/0) to E_c. Tentative assignments are indicated by ?.

Defect	E(0/+) Calc.	E(0/+) Observed	E(-/0) Calc.	E(-/0) Observed	Ref.
VP	0.2	-	0.45	0.43
VO	0.0	-	0.13	0.18
V2 (C2h)	0.42	0.23	0.51	0.43
C-CH	0.24	?	0.20	0.20
CiP	0.36	0.48	0.60	0.38
H$_{\rm BC}$	0.94	1.0	-	-
H$_{\rm AB}$	-	-	0.78	?
V2O	-	-	0.47	-
Ag	0.46	0.37	0.60	0.56	ref. [#!icds97-2!#]
AgH	0.36	0.28 (H2) ?	0.45	0.45 (E2) ?	ref. [#!icds97-2!#]
AgH2	0.33	0.38 (H3)?	0.50	-	ref. [#!icds97-2!#]
AgH3	0.0	-	0.13	-
AgH4	0.0	-	0.97	-
Au	0.21	0.35	0.66	0.54	ref. [#!svein-95!#]
AuH	0.36	0.21 (G2)?	0.62	$\sim$ 0.53 (G4)?	ref. [#!svein-95!#]
AuH2	0.28	0.47 (G3) ?	0.62	-	ref. [#!svein-95!#]
AuH3	0.0	-	0.26	-
AuH4	-	-	1.4	-
Pd	0.52	0.31	0.22	0.23	ref. [#!sachse3-97!#]
PdH	0.53	0.55 (H280)?	0.39	0.29 (E160) ?	ref. [#!sachse3-97!#]
PdH2	0.0	-	0.39	0.43 (E200) ?	ref. [#!sachse3-97!#]
PdH3	0.0	-	1.0	1.0 (H140) ?	ref. [#!sachse3-97!#]
PdH4	-	-	0.53	-
Pt	0.23	0.35	0.36	0.23	ref. [#!sachse2-97!#]
PtH	0.52	0.4 (H210) ?	0.42	-	ref. [#!sachse2-97!#]
PtH2	0.0	-	0.45	0.50 (E250)?
PtH3	0.0	?	1.4	0.9 (H150)	ref. [#!sachse2-97!#]
PtH4	-	-	1.0	-

 

Table 2: (-/-) levels of deep centres, relative to E_c, in eV. Tentative assignments are indicated by ?.

Defect	E(-/-) Calc.	E(-/-) Obs.	Ref.
V2	0.35	0.23 ?
AgH	0.36	0.09 (E3) ?	ref. [#!icds97-2!#]
AuH	0.22	0.19 (G1) ?	ref. [#!davidson-96!#]
AgH2	0.0	-
AuH2	0.0	-
PdH	$\sim$ 0.0	-
PdH2	$\sim$ 0.0	-
PtH	0.12	-
PtH3	0.0	-