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Lieu
In place of
Use 'lieu' to mean 'instead of' when you're talking about replacements or alternatives. 
Formal situations
'In lieu of' is fairly formal; it's used in business, legal contexts, or serious writing more than casual conversation. 
Fixed expression
'In lieu of' is a set phrase; don't change its form. Don't say 'in the lieu of' or 'in lies of'. 