To cite lmerTest in publications use:

Kuznetsova A, Brockhoff PB, Christensen RHB (2017). “lmerTest Package: Tests in Linear Mixed Effects Models.” Journal of Statistical Software, 82(13), 1–26. doi:10.18637/jss.v082.i13.

Corresponding BibTeX entry:

  @Article{,
    title = {{lmerTest} Package: Tests in Linear Mixed Effects Models},
    author = {Alexandra Kuznetsova and Per B. Brockhoff and Rune H. B.
      Christensen},
    journal = {Journal of Statistical Software},
    year = {2017},
    volume = {82},
    number = {13},
    pages = {1--26},
    doi = {10.18637/jss.v082.i13},
  }