[R] What is the default covariance structure in the glmmPQL function (MASS package)?

[Gerda Börner]

Hello,

currently I am trying to fit a generalized linear mixed model using the 
glmmPQL function in the MASS package. I am working with the data 
provided by the book from Heck, Thomas and Tabata (2012) - 
https://www.routledge.com/Multilevel-Modeling-of-Categorical-Outcomes-Using-IBM-SPSS/Heck-Thomas-Tabata/p/book/9781848729568

I was wondering, which variance-covariance structure the glmmPQL 
function is using by default and if it is possible to vary the 
variance-covariance structure with the glmmPQL function. Unfortunately I 
couldn't manage to find out myself.
If it is possible to change it, could someone tell me how to do so? I am 
especially interested in a diagonal structure versus an unstructured 
variance-covariance structure.

Thanks a lot in advance.

Gerda